US9825229B2 - Purification of carbon nanotubes via selective heating - Google Patents
Purification of carbon nanotubes via selective heating Download PDFInfo
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- US9825229B2 US9825229B2 US14/772,312 US201414772312A US9825229B2 US 9825229 B2 US9825229 B2 US 9825229B2 US 201414772312 A US201414772312 A US 201414772312A US 9825229 B2 US9825229 B2 US 9825229B2
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- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
- H10K85/20—Carbon compounds, e.g. carbon nanotubes or fullerenes
- H10K85/221—Carbon nanotubes
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- H01L51/0048—
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
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- B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
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- C01B31/026—
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- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B32/00—Carbon; Compounds thereof
- C01B32/15—Nano-sized carbon materials
- C01B32/158—Carbon nanotubes
- C01B32/168—After-treatment
- C01B32/17—Purification
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/02104—Forming layers
- H01L21/02365—Forming inorganic semiconducting materials on a substrate
- H01L21/02518—Deposited layers
- H01L21/02587—Structure
- H01L21/0259—Microstructure
- H01L21/02606—Nanotubes
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- H01L51/0004—
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- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/139—Processes of manufacture
- H01M4/1393—Processes of manufacture of electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K71/00—Manufacture or treatment specially adapted for the organic devices covered by this subclass
- H10K71/10—Deposition of organic active material
- H10K71/12—Deposition of organic active material using liquid deposition, e.g. spin coating
- H10K71/13—Deposition of organic active material using liquid deposition, e.g. spin coating using printing techniques, e.g. ink-jet printing or screen printing
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B2202/00—Structure or properties of carbon nanotubes
- C01B2202/20—Nanotubes characterized by their properties
- C01B2202/22—Electronic properties
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B2202/00—Structure or properties of carbon nanotubes
- C01B2202/20—Nanotubes characterized by their properties
- C01B2202/34—Length
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- H01L51/0025—
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K71/00—Manufacture or treatment specially adapted for the organic devices covered by this subclass
- H10K71/311—Purifying organic semiconductor materials
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- Carbon nanotubes are allotropes of carbon comprising one or more cylindrically configured graphene sheets and are classified on the basis of structure as either single walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs). Typically having small diameters ( ⁇ 1-30 nanometers) and large lengths (up to several microns), SWNTs and MWNTs commonly exhibit length to diameter ratios of ⁇ 10 2 to about 10 7 . Carbon nanotubes exhibit either metallic or semiconductor electrical behavior, and the energy band structure of nanotube materials varies considerably depending on their precise molecular structure and diameter.
- SWNTs single walled carbon nanotubes
- MWNTs multiwalled carbon nanotubes
- Carbon nanotubes exhibit either metallic or semiconductor electrical behavior, and the energy band structure of nanotube materials varies considerably depending on their precise molecular structure and diameter.
- SWNTs Single walled carbon nanotubes
- SWNTs are made up of a single, contiguous graphene sheet joined with itself to form a hollow, seamless tube, in some cases with capped ends similar in structure to smaller fullerenes.
- SWNTs typically have very small diameters ( ⁇ 1 nanometer) and are often present in curled, looped and bundled configurations.
- SWNTs are chemically versatile materials capable of functionalization of their exterior surfaces and encapsulation of materials within their hollow cores, such as gases or molten materials.
- SWNTs are believed to have remarkable mechanical properties, such as tensile strengths at least 50 times that of steel.
- the electron transport behavior in SWNTs is predicted to be essentially that of a quantum wire, and the electrical properties of SWNTs have been observed to vary upon charge transfer doping and intercalation, opening up an avenue for potentially tuning the electrical properties of nanotube materials.
- SWNTs have also been demonstrated to have very high intrinsic field affect mobilities (e.g., about 10,000 cm 2 V ⁇ 1 s ⁇ 1 ) making them interesting for possible applications in nanoelectronics.
- SWNT-based electronic device platform Despite substantial progress in developing a SWNT-based electronic device platform, several factors impede commercialization of these systems.
- Second, films comprising SWNT networks are typically a mixture of metallic tubes and semiconducting tubes.
- the presence of metallic tubes in the network often results in a significant extent of purely metallic conductive pathways between the source/drain (S/D) electrodes of SWNT-based thin-film transistor (TFT) devices.
- S/D source/drain
- TFT thin-film transistor
- the present invention provides device component geometries and fabrication strategies for enhancing the electronic performance of electronic devices based on thin films of semiconducting carbon nanotubes.
- the invention provides, for example, processes utilizing selective heating of a subset of carbon nanotubes having one or more selected physical and/or chemical properties so as to selectively separate nanotubes of the subset from a precursor layer containing nanotubes, such as a layer containing substantially aligned carbon nanotubes.
- a flow of a thermocapillary resists is generated via selective heating of carbon nanotubes so as to expose nanotubes having one or more selected physical and/or chemical properties, thereby allowing subsequent separation from the precursor layer, for example via removal or transfer processes.
- Examples of processes useful in the present invention for achieving selective heating and separation of carbon nanotubes based on one or more selected physical and/or chemical properties include absorption of electromagnetic radiation (e.g., via laser or microwave source), application of an electromagnetic field, electric field, magnetic field and/or application or a voltage. In some embodiments, for example, one or more of the following are applied simultaneously, electromagnetic radiation, electromagnetic field, electric field, magnetic field and/or application or a voltage.
- devices and methods of the present invention provide for the purification of mixtures of metallic and semiconducting carbon nanotubes.
- mixtures of substantially aligned single-walled carbon nanotubes may be purified by methods involving selective heating of one class of nanotubes, which may be separated from the mixture.
- metallic nanotubes are selectively heated to induce flow of a thermocapillary resist away from the metallic nanotubes to expose the metallic nanotubes while semiconducting nanotubes remain covered by the resist. Separation of the exposed and unexposed nanotubes may be carried out, for example, by etching, ablation, transfer printing, and other known methods.
- additional processing may be carried out to protect the exposed nanotubes followed by removal of the thermocapillary resist, and optionally the nanotubes underlying the thermocapillary resist.
- the present purification methods are versatile, thereby providing a platform enabling improved nanotube-based electronic devices and systems well suited for a range of device applications, including thin film electronics, large area electronics (e.g., macroelectronics), flexible electronics, and sensing.
- Methods and devices of the present invention are compatible with low temperature processing and assembly on a wide range of device substrates, including mechanically flexible substrates such as polymer substrates.
- Processing methods and design strategies of the present invention are complementary to conventional microfabrication and nanofabrication platforms, and can be effectively integrated into existing photolithographic, etching and thin film deposition patterning strategies, systems and infrastructure.
- methods of the present invention enable low cost fabrication of high performance nanotube based semiconductor devices, such as thin film transistors, transistor arrays, and integrated electronic circuits.
- a method for purifying a layer of carbon nanotubes comprises: providing a precursor layer of substantially aligned carbon nanotubes supported by a substrate, wherein the precursor layer comprises a mixture of first carbon nanotubes and second carbon nanotubes; covering the precursor layer of carbon nanotubes with a thermocapillary resist, wherein the thermocapillary resist is in thermal contact with at least a portion of the carbon nanotubes; selectively heating the first carbon nanotubes, thereby causing thermocapillary flow of the thermocapillary resist away from the first carbon nanotubes to expose the first carbon nanotubes; and separating the first carbon nanotubes from the second carbon nanotubes, thereby generating a purified layer of carbon nanotubes.
- a method for making an electronic device comprises: providing a precursor layer of substantially aligned carbon nanotubes supported by a substrate, wherein the precursor layer comprises a mixture of first carbon nanotubes and second carbon nanotubes; covering the precursor layer of carbon nanotubes with a thermocapillary resist, wherein the thermocapillary resist is in thermal contact with at least a portion of the carbon nanotubes; selectively heating the first carbon nanotubes, thereby causing thermocapillary flow of the thermocapillary resist away from the first carbon nanotubes to expose the first carbon nanotubes; separating the first carbon nanotubes from the second carbon nanotubes, thereby generating a purified layer of carbon nanotubes; and providing one or more device component structures in electrical or physical contact with the purified layer of carbon nanotubes, thereby making the electronic device.
- first carbon nanotubes can be a first set of nanotubes, a first class of nanotubes, or a first distribution of nanotubes.
- Second carbon nanotubes can be a second set of nanotubes, a second class of nanotubes, or a second distribution of nanotubes.
- Each type, set, class or distribution of nanotubes may be characterized by an electronic, optical, physical, chemical or other property in common.
- the precursor layer comprises an array of substantially longitudinally aligned carbon nanotubes, such as an array generated using a guide growth substrate or guided deposition substrate.
- the selective heating results from absorption of electromagnetic radiation, electronic resistance, mobility, direct thermal contact or electromagnetic induction.
- Carbon nanotubes of the present invention may be single walled carbon nanotubes, multiwalled carbon nanotubes or a mixture of both. Use of single walled nanotubes (SWNTs) is preferred for some applications given their particularly useful semiconducting properties.
- the precursor layer is a monolayer or sub-monolayer of carbon nanotubes.
- the terms “nanotube surface concentration” and “nanotube density” are used interchangeably and refer to the number of nanotubes per area of substrate having the nanotubes.
- carbon nanotubes of the precursor layer are a mixture of semiconducting nanotubes and metallic nanotubes, wherein there are more semiconducting nanotubes than metallic nanotubes.
- the first carbon nanotubes may be metallic carbon nanotubes and the second carbon nanotubes may be semiconducting carbon nanotubes.
- Conventional sources of carbon nanotubes such as SWNTs, typically generate mixtures having more semiconducting nanotubes than metallic nanotubes, for example mixtures having between 60-80% semiconducting nanotubes and 40-20% metallic nanotubes or mixtures having between 65-75% semiconducting nanotubes and 35-25% metallic nanotubes.
- carbon nanotubes of the precursor layer are a mixture of semiconducting nanotubes and metallic nanotubes, wherein there are more semiconducting nanotubes than metallic nanotubes, for example a mixture wherein there are at least 1.5 times more semiconducting nanotubes than metallic nanotubes, and in some embodiments wherein there are 1.5-4 times more semiconducting nanotubes than metallic nanotubes.
- Carbon nanotubes of the precursor layer can be generated by a range of synthetic methods including, chemical vapor deposition, pyrolysis, arc discharge, catalytic methods and laser ablation methods.
- Precursor layers of carbon nanotubes of the present invention may further comprise additional components, such as dopants or components enhancing the mechanical properties of the nanotube layer.
- a precursor layer of the present invention comprises one or more carbon nanotubes in a selected geometry.
- the precursor layer may be provided by growing substantially aligned carbon nanotubes on a substrate comprising a guided growth substrate or by printing substantially aligned carbon nanotubes onto a substrate.
- Suitable methods for printing the substantially aligned carbon nanotubes may for example be selected from ink jet printing, thermal transfer printing, contact printing, dry transfer printing or screen printing.
- the carbon nanotubes of the precursor layer generally have an average length selected from a range of 20 nanometers to 100 microns, or 50 nanometers to 10 microns, or 75 nanometers to 1 micron, or 100 nanometers to 500 nanometers. Further, an average spacing between adjacent carbon nanotubes of the precursor layer is typically selected from a range of 2 nm to 100 ⁇ m, or 5 nm to 10 ⁇ m, or 10 nm to 1 ⁇ m, or 20 nm to 500 nm, or 30 nm to 250 nm. In an embodiment, an average spacing between adjacent carbon nanotubes of the precursor layer is at least 2 nm, or at least 5 nm or at least 10 nm, or at least 20 nm, or at least 30 nm.
- a surface concentration of carbon nanotubes of the precursor layer is typically selected from a range of 0.2 carbon nanotubes micron ⁇ 2 to 100 carbon nanotubes micron ⁇ 2 , or 0.5 carbon nanotubes micron ⁇ 2 to 50 carbon nanotubes micron ⁇ 2 , or 1 carbon nanotube micron ⁇ 2 to 25 carbon nanotubes micron ⁇ 2 , or 2 carbon nanotubes micron ⁇ 2 to 15 carbon nanotubes micron ⁇ 2 .
- the precursor layer of substantially aligned carbon nanotubes includes less than 100 carbon nanotube crossings per square micron, or less than 50 carbon nanotube crossings per square micron, or less than 25 carbon nanotube crossings per square micron, or less than 10 carbon nanotube crossings per square micron.
- the layer of substantially aligned carbon nanotubes typically has a thickness less than or equal to 10 nanometers, or less than or equal to 5 nanometers, or less than or equal to 2 nanometers.
- the layer of substantially aligned carbon nanotubes is a monolayer film or a substantially monolayer film.
- thermocapillary resists for covering the nanotubes, and optionally a portion of a substrate, include but are not limited to thermocapillary resists having a room temperature viscosity selected from a range of 0.5 Pa ⁇ s to 50 Pa ⁇ s, or 1 Pa ⁇ s to 30 Pa ⁇ s, or 3 Pa ⁇ s to 15 Pa ⁇ s.
- the thermocapillary resist comprises a conformal layer on and between the carbon nanotubes, where the thermocapillary resist is in physical contact with the carbon nanotubes.
- the thermocapillary resist comprises a substantially uniform layer.
- the substantially uniform layer may, for example, have a thickness selected from a range of 1 nm to 10 ⁇ m, or 5 nm to 1 ⁇ m, or 10 nm to 500 nm, or 10 nm to 50 nm, or a thickness less than or equal to 500 nm, or less than or equal to 250 nm, or less than or equal to 100 nm, or less than or equal to 50 nm.
- the substantially uniform layer is a continuous layer.
- the thermocapillary resist comprises a molecular organic species.
- the thermocapillary resist may be selected from the group consisting of ⁇ , ⁇ , ⁇ ′-Tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene polymethylmethacrylate (PMMA), polystyrene (PS), and low molecular weight silicon containing polymers such as poly(styrene-dimethylsiloxane) (PS-PDMS); oligosaccharide-trimethylsilylstyrene, and polyhedral oligomeric silsesquioxane (FOSS).
- the thermocapillary resist comprises a molecular weight selected from a range of 50 g/mol to 1000 kg/mol, or 500 g/mol to 500 kg/mol, or 1 kg/mol to 100 kg/mol.
- At least 1%, 5%, 10%, 20%, 50% or 75% of a lateral cross section of each of the first carbon nanotubes is exposed by the step of selectively heating the first carbon nanotubes.
- the step of separating the first carbon nanotubes from the second carbon nanotubes may include: etching exposed carbon nanotubes, transfer printing the exposed carbon nanotubes, transfer printing the carbon nanotubes covered by the thermocapillary resist, removing the exposed carbon nanotubes from the precursor layer, or removing the covered carbon nanotubes from the precursor layer (e.g., by performing steps of applying a protective layer to the exposed nanotubes, removing the thermocapillary resist and separating the previously covered carbon nanotubes from the precursor layer).
- any of the methods described herein may include a step of removing the thermocapillary resist after the step of separating.
- Carbon nanotubes and devices or device components of the present invention may be supported by a substrate.
- Useful substrates for supporting the nanotubes, devices and device components of the present invention include but are not limited to mechanically flexible substrates such as polymer substrates, rigid substrates, dielectric substrates, metal substrates, ceramic substrates, glass substrates, semiconductor substrates and functional substrates prepatterned with one or more device components.
- the present invention also includes devices and device components provided on (i.e. supported by) contoured substrates, including curved substrates, curved rigid substrates, concave substrates, and convex substrates.
- the step of selectively heating a type, set, class or distribution of nanotubes may be carried out by application of electromagnetic energy, current, an electric field, a magnetic field, microwave energy or laser radiation to the nanotubes of the precursor layer.
- selective heating comprises absorption of energy by metallic carbon nanotubes, wherein the energy is insufficient to overcome the Schottky barrier of semiconducting carbon nanotubes.
- a ratio of a temperature increase of the first carbon nanotubes to a temperature increase of the second carbon nanotubes during the step of selective heating is 1.25 or greater, or 1.7 or greater, or 2 or greater, or 2.5 or greater.
- an average temperature gradient within the thermocapillary resist proximate to the first carbon nanotubes is at least 1 K/ ⁇ m, or at least 1.5 K/ ⁇ m, or at least 2 K/ ⁇ m.
- the selective heating is provided by one or more of an optical source, a microwave source, a laser source, a DC source, an AC source, or an acoustic source.
- the source may be pulsed or continuous.
- selective heating of the nanotubes occurs in a transistor configuration, where the carbon nanotubes of the precursor layer are in electrical contact with a source electrode, a drain electrode and a gate electrode.
- the selective heating is provided by one or more of a DC source and an AC source delivering a power per carbon nanotube selected from a range of 5 ⁇ W/ ⁇ m/tube to 50 ⁇ W/ ⁇ m/tube, or 10 ⁇ W/ ⁇ m/tube to 30 ⁇ W/ ⁇ m/tube, to the source electrode.
- the source is pulsed at a frequency selected from a range of CW (i.e., 0 Hz) to 100 MHz, or CW (i.e., 0 Hz) to 10 MHz, or CW (i.e., 0 Hz) to 1 MHz, or CW (i.e., 0 Hz) to 500 Hz, and the source is activated for a duration selected from a range of 1 ns to 100 minutes, or 10 ns to 10 minutes, or 100 ns to 1 minute.
- CW i.e., 0 Hz
- selective heating of the nanotubes occurs via laser irradiation using a laser source producing radiation having an energy selected from a range of 100 nJ to 100 mJ, or 1 ⁇ J to 10 mJ and a power less than 1 kJ/m 2 , or less than 0.5 kJ/m 2 .
- the laser source produces radiation having a wavelength selected from a range of 1 ⁇ m to 10 ⁇ m, or 1.5 ⁇ m to 8 ⁇ m, or 2 ⁇ m to 5 ⁇ m.
- the laser source may be pulsed at a frequency selected from a range of 1 Hz to 100 MHz, or 1 Hz to 1000 KHz or 1 Hz to 100 KHz, or 1 Hz to 10 KHz.
- the laser source is activated for a duration selected from a range of 1 nm to 300 minutes, or 10 ns to 200 minutes, or 100 ns to 100 minutes, or 1 ⁇ s to 10 minutes, or 1 ms to 1 minute.
- selective heating of the nanotubes occurs in a microwave configuration, where the carbon nanotubes of the precursor layer are in electromagnetic communication with at least one microwave antennae.
- the electromagnetic communication may involve physical contact, electrical contact or both physical and electrical contact. In an embodiment, the electromagnetic communication does not comprise physical contact.
- the microwave source produces radiation having an energy selected from a range of 50 J/sec to 10 kJ/sec, or 100 J/sec to 1 kJ/sec.
- the microwave source is pulsed at a frequency selected from a range of CW (i.e., 0 Hz) to 100 MHz, or CW (i.e., 0 Hz) to 10 MHz, or CW (i.e., 0 Hz) to 1 MHz, or CW (i.e., 0 Hz) to 500 Hz.
- the microwave source is activated for a duration selected from a range of 0.1 ⁇ s to 300 minutes, or 1 ⁇ s to 300 minutes, or of 0.1 ⁇ s to 100 minutes or 1 ⁇ s to 100 minutes, or 1 ms to 10 minutes.
- the selective heating comprises differential absorption of a preselected wavelength of radiation between the first carbon nanotubes and the second carbon nanotubes, wherein the first carbon nanotubes absorb more than 1.25 times, or more than 1.5 times as much energy as the second carbon nanotubes.
- the preselected wavelength may, for example, be selected from a range of 100 nm to 20 ⁇ m, or 300 nm to 20 ⁇ m, or 0.5 ⁇ m to 10 ⁇ m, or 1 ⁇ m to 5 ⁇ m.
- selective heating of the nanotubes occurs in a two-terminal configuration, where the first and second carbon nanotubes of the precursor layer are in electrical contact with a first electrode and a second electrode.
- An electrode bias voltage between the first electrode and the second electrode may, for example, be selected from a range of 0.01 V to 500 V, or 0.1 V to 500 V, or 0.01 V to 100 V, or 1 V to 50 V, or 5 V to 25 V.
- the first and second electrodes are interdigitated.
- the selective heating may be provided, for example, by one or more of a DC source and an AC source delivering a current selected from a range of 0.01 mA to 100 A, or 0.1 mA to 10 A, or 1 mA to 1 A, or 10 mA to 1 A.
- the DC or AC source is pulsed at a frequency selected from a range of 1 Hz to 500 MHz, or 1 Hz to 100 MHz, or 10 Hz to 500 MHz, or 100 Hz to 50 MHz, or 1 MHz to 5 MHz.
- the DC or AC source is activated for a duration selected from a range of 0.1 ⁇ s to 100 minutes, or 1 ⁇ s to 10 minutes, or 1 ms to 1 minute.
- a method for purifying a layer of carbon nanotubes comprises: providing a precursor layer of substantially aligned carbon nanotubes supported by a substrate, wherein the precursor layer comprises a mixture of first carbon nanotubes and second carbon nanotubes; and selectively heating the first carbon nanotubes to separate the first carbon nanotubes from the second carbon nanotubes, thereby providing a purified layer of carbon nanotubes comprising at least 50%, 60%, 70%, 80%, or 90% of the first or the second carbon nanotubes.
- the present invention provides a transistor wherein the nanotube layer provides a semiconductor channel between first and second electrodes comprising source and drain electrodes.
- Transistors of the present invention may further comprise a gate electrode and dielectric layer; wherein the dielectric layer is provided between the gate electrode and the precursor layer.
- the gate electrode is electrically isolated from, and positioned close enough to, the semiconductor channel such that electron transport through the channel is modulated by application of an electric potential to the gate electrode.
- the layer has a strip geometry and comprises a plurality of strips of interconnected carbon nanotube networks, wherein strips of interconnected carbon nanotubes extend lengths from source to drain electrodes and are aligned in the electron transport direction of the transistor, optionally in a parallel strip orientation.
- a transistor of this aspect is a thin film transistor.
- a transistor of this aspect has an on/off ratio greater than or equal to 100, and preferably for some applications greater than or equal to 1000.
- a transistor of this aspect has a field effect mobility greater than or equal to 0.1 cm 2 V ⁇ 1 s ⁇ 1 , and preferably for some applications a field effect mobility greater than or equal to 10 cm 2 V ⁇ 1 s ⁇ 1 .
- the invention provides nanotube-based transistor arrays and integrated circuits comprising a plurality of nanotube-based transistors.
- the invention provides an electronic device comprising: a first electrode; a second electrode; and a layer of substantially aligned carbon nanotubes positioned between and in electrical contact with the first electrode and the second electrode, wherein the layer of substantially aligned carbon nanotubes comprises at least 90% semiconducting carbon nanotubes.
- fabrication of the electronic device does not include a fluidic assembly step.
- Electronic devices of the present invention include a range of nanotube-based devices.
- Electronic devices of the present invention include for example, transistors, diodes, light emitting diodes, integrated circuits and photodetectors comprising one or more layers of carbon nanotubes.
- the first electrode may be a source electrode
- the second electrode may be a drain electrode
- the layer of substantially aligned semiconducting carbon nanotubes may be a semiconductor channel of the transistor.
- the layer of substantially aligned semiconducting carbon nanotubes provides a semiconductor channel between first and second electrodes, wherein the semiconductor channel has a length selected from a range of 50 nanometers and 1000 microns.
- contact printing of carbon nanotubes is achieved using soft lithography methods, such as dry transfer printing techniques using a conformable transfer device such as an elastomeric stamp.
- FIG. 1A provides a schematic flow diagram illustrating a method of the present invention for purifying carbon nanotubes, for example, by separation of a first class of nanotubes having one or more selected physical or chemical properties from other nanotubes in a nanotube precursor layer.
- FIG. 1B provides a schematic flow diagram corresponding to a method of the present invention further illustrating various process approaches for selective heating the first carbon nanotubes having one or more selected physical or chemical properties.
- FIG. 1C provides a schematic flow diagram corresponding to a method of the present invention further illustrating various process approaches for selective removal of the first carbon nanotubes that undergo selective heating.
- FIG. 1D provides a schematic flow diagram corresponding to a method of the present invention further illustrating various process approaches for generating a precursor layer comprising substantially aligned carbon nanotubes.
- FIG. 2( i ) Thermocapillary effects provide the basis for a process that enables selective and complete elimination of m-SWNTs from electronically heterogeneous arrays of SWNTs.
- (a,b) Schematic illustration and corresponding atomic force microscope images of various stages of the process applied to an array of five m-SWNTs and three s-SWNTs. Uniform thermal evaporation forms a thin, amorphous organic coating that functions as a thermocapillary resist.
- a series of processing steps defines a collection of electrodes and dielectric layers for selective injection of current into the m-SWNTs.
- thermocapillary resist that results from this process induces thermal gradients that drive flow of thermocapillary resist away from the m-SWNT, to form open trenches with widths, measured near the substrate, of ⁇ 100 nm. Reactive ion etching physically eliminates the m-SWNT exposed in this fashion, while leaving the coated s-SWNTs unaltered. Removing the thermocapillary resist and electrode structures completes the process, to yield arrays comprised only of s-SWNTs. (c) Typical transfer characteristics for a transistor built with an array of SWNTs in a partial gate geometry, evaluated before and after purification. The quantities I on,b /I on,a correspond to currents measured in the on states before and after purification, respectively.
- the on/off ratio improves by 2 ⁇ 10 4 times, while I on,a /I on,b remains relatively large, i.e. ⁇ 0.25.
- Nanoscale thermal transport associated with Joule heating in SWNTs leads to thermal gradients that are sufficiently large to drive thermocapillary flows in thin organic coatings.
- thermocapillary resist topographical images of the same device shown in the scanning Joule expansion micrograph of (a), coated with a thin ( ⁇ 25 nm) layer of thermocapillary resist, collected after operation at bias conditions of 0.27 (top), 0.5 (middle) and 1.0 V/ ⁇ m (bottom). Comparison of these images to those collected by scanning Joule expansion microscopy reveals a clear correlation between AC expansion (E 0 ; and, therefore, temperature) and formation of trenches in the thermocapillary resist (DC heating).
- the results indicate small increases in temperature for levels of Joule heating that induce trenches in the thermocapillary resist ( ⁇ 3-10 ⁇ W/ ⁇ m).
- the top graph shows AC thermal expansion (arbitrary units) measured by scanning Joule expansion microscope along the length (y) of the fourth SWNT from the left in the array that appears in (a) and (b).
- the bottom graph shows the width of the corresponding trench that appears in the thermocapillary resist (W Tc measured at the top of the film) for an applied bias of ⁇ 1 V/ ⁇ m.
- the results show variations in W Tc that are nearly ten times smaller than those in expansion (and therefore temperature).
- Measurements of the average W Tc as a function of Q 0 The results reveal no systematic dependence on Q 0 over this range. The highlighted region corresponds to the values of Q 0 associated with optimized conditions for the purification process.
- FIG. 3 Experimental and theoretical studies reveal essential aspects of nanoscale thermocapillary flows in thin organic coatings on heated SWNTs.
- the simulations used polystyrene due to availability of relevant materials parameters in the literature.
- FIG. 4 Thermocapillary effects can be used to achieve purely semiconducting arrays of SWNTs in strategies that scale to large areas.
- (a) Optical microscope image of a set of electrodes for thermocapillary purification of an array of many hundreds of SWNTs.
- (c) Transfer characteristics before and after removal of m-SWNTs from the region between the electrodes shown in (a). The results indicate outcomes consistent with observations of small-scale demonstrations, i.e. high on/off ratios ⁇ 1 ⁇ 10 3 and modest reductions in on current (I on, a /I on, b ⁇ 20%).
- (d) Optical micrograph and schematic illustration of alternative mode for scaled implementation. Here, an interconnected array of 25 sets of electrodes allows purification over a collection of small regions, in a parallel fashion. Associated transfer curves are similar to those shown in (c).
- FIG. 5 A re-usable bottom electrode structure reduces the number of processing steps needed for thermocapillary purification.
- a Schematic illustration of two purification processes implemented on different arrays of SWNTs using a single bottom split gate electrode structure
- i As-grown array of aligned SWNTs
- ii bottom electrode after transfer of these SWNTs
- gate electrode gold
- s-SWNTs that remain after purification
- iv transfer of the s-SWNTs to a device substrate.
- FIG. 6 Purified arrays of s-SWNTs can be used in short channel transistors and logic gates.
- FIG. 7 Critical power for trench formation. Summary of results on experimental investigations of trench formation (5 min, 60° C.) in devices with single or several SWNT, in which an individual SWNT contributed a majority of the current. The findings define the critical power density to form trenches. All experiments were performed with s-SWNT in their “off” state (+20 V G ). (a) Scatter plot of device-level power density associated with the experiments. All devices with power density below 3.3 ⁇ W/ ⁇ m show no trenches (blue), all devices with power density above 10 ⁇ W/ ⁇ m show complete trenches along the entirety of their length (red), while those with intermediate powers show trenches along part of their length (yellow).
- FIG. 8 Tc-resist characterization.
- FIG. 9 SWNT resistivity data.
- c,d Histograms showing distributions of individual SWNT resistivities for two data sets, one based on back-gated devices on SiO 2 /Si and the other based on top-gated devices with a gate dielectrica of SOG/HfO 2 . Distributions representative of previously published results on arrays of SWNT.
- FIG. 11 Summary of SJEM measurements.
- FIG. 12 Thermal modeling geometry.
- FIG. 13 Time dependent trench formation study. 3 ⁇ 3 ⁇ m AFM images associated with in-situ measurement of trench formation. Brief intervals of bias were applied and the associated topography was measured (30 s scans) in between each interval. These images are associated with various total accumulated bias durations as trenches evolve.
- FIG. 14 Details of trench width identification.
- the trench edges were associated with the peaks of the pileup on the trench edge.
- that width of the trench at the base is substantially narrower than these values. AFM tip artifacts make precise measurement of this inner width difficult.
- FIG. 15 Time dependence of trench width.
- FIG. 16 Thermocapillary flows in Tc-resist studied with calibrated, heated AFM tips.
- General behavior is consistent with that observed for SWNT Joule heating.
- features form in the Tc-resist even at low temperature rises. Feature sizes increase with time.
- FIG. 17 Pulsed biases for forming trenches in large and/or high density arrays of SWNTs.
- V GS +20V DC, 60 sec total stress duration, 50° C. background heating.
- For 10% duty cycle clearly defined trenches are observed.
- FIG. 18 Behaviors in other candidate materials for Tc-resists.
- FIG. 19 Theoretical trench width for processes based on critical temperatures.
- FIG. 20 Trenches in cases with neighboring SWNTs.
- FIG. 21 Details of TcEP in a parallel operational mode.
- FIG. 22 Details of inverter fabrication
- (a) Optical micrographs corresponding to process steps for inverter fabrication. TcEP was performed on two arrays in parallel, the gate electrode and dielectric layers were removed, and then new, top-gated TFTs were fabricated with appropriate channel lengths for optimal inverter performance.
- (b) I-V characteristics of driver FETs associated with electrodes used for TcEP (30 ⁇ m channel length) and final device configuration (3.5 ⁇ m channel length), respectively.
- FIG. 23 Inverter fabricated using arrays processed by TcEP. (a) load line analysis for the inverter and (b) voltage transfer characteristics measured and predicted from load line analysis.
- FIG. 24 Modeling results used to extract mobilities from device characteristics.
- (a) Cross-sectional schematic illustration of a partial gate device consisting of source/drain (Ti/Pd), gate (Ti), and a gate dielectric of PVA/Al 2 O 3 /Spin-on-glass (SOG). Values for the width (W), length (L) and thickness (T) of different components of the device are also specified.
- FIG. 25 SEM and AFM images at each stage of the TcEP process, as implemented with a BSGS.
- the red arrow highlights a pair of s-SWNTs throughout this process.
- FIG. 26 Demonstration of reusability of the BSGS.
- FIG. 27 Off state stability for top split gate structures.
- FIG. 29 Full characterization of split gate devices.
- (a) Transfer characteristics for a device with W/L 1000/30 ⁇ m after TcEP, evaluated at various drain bias (V DS ) conditions
- FIG. 30 Full characterization of split gate devices.
- (a) Transfer characteristics for two interconnected, purified devices to form effective channel dimensions of W/L 2000/30 ⁇ m, at low drain bias.
- FIG. 32 Simulation and experimental results on trends in on/off ratio with bias condition.
- (a): Experimental I DS -V GS characteristics for different drain bias at (a) L OV 5 ⁇ m and (b) 30 ⁇ m.
- (c) Experimental (solid symbol) on/off current ratio vs V DS for different L OV follows simulation (open symbols are simulated for L OV 5, 20 ⁇ m). Dotted lines are guide to eye only.
- BTBT band-to-band tunneling
- c Two-dimensional potential profile (color contours) in off-state along the surface of a quartz wafer (directions X and Z are along and across the nanotube, respectively) containing the nanotube.
- FIG. 34 Schematic showing exemplary purification process involving thermocapillary flow.
- FIG. 35 (a) Schematic of thermocapillary based purification process; (b) Scalability questions. Split gate structure need poses a barrier to wafer scale deployment. Clearly an e-field exposure based approach would provide scalability.
- FIG. 36 NTF process driven by microwave irradiation.
- FIG. 37 Microwave antenna structure.
- FIG. 38 Two developed microwave irradiation geometries.
- FIG. 39 Current Equivalent Circuit model.
- FIG. 40 Metal electrode performance.
- FIG. 41 Selectivity as a function of metal type.
- FIG. 42 Transport barrier picture.
- FIG. 43 Selectivity with Mo contacts.
- FIG. 44 Deep trench formation in Mo. Clearly we “bottom out” the trenches suggesting we can lower power of processing time (bottom).
- FIG. 45 Process flow for non-contact microwave case.
- FIG. 46 NTF process driven by microwave irradiation: contact case vs. non-contact case.
- FIG. 47 Structure of removable antenna structure.
- FIG. 48 Model for the non-contact microwave case.
- FIG. 49 Work function dependence; non-contact case.
- FIG. 50 Antenna array strategy to achieve large area irradiation.
- FIG. 51 Trenched generated via an antenna array.
- FIG. 52 Process flow Laser initiated heating of metallic nanotubes.
- FIG. 53 Absorption properties of SWNT.
- FIG. 54 Laser baser exposure set-up. Diffraction limited excitation beam is raster scanner over the nanotubes.
- FIG. 55 AFM imager of laser irradiated substrate. Trenches are observed.
- FIG. 56 A series of laser irradiated samples. Selectivity is statically consistent but was additionally confirmed via Raman analysis.
- FIG. 57 Transfer characteristics of devices.
- FIG. 58 Process flow of direct laser ablation of metallic nanotubes.
- FIG. 59 In-situ ablation apparatus.
- FIG. 60 SEM of ablated die area.
- FIG. 61 Ablation power/wavelength dependence.
- FIG. 62 AFM topography of CW irradiated TcEP substrate. There is no trench formation indicated even at full power.
- FIG. 63 MG2OH our first generation TcEP resist.
- FIG. 64 Model derived dependence of trench properties on viscoelastic parameters and heating power.
- FIG. 65 Process flow for removing metallic SWNTs via a microwave contact approach.
- FIG. 66 Process flow for removing metallic SWNTs via a microwave noncontact approach.
- FIG. 67 Process flow for removing metallic SWNTs via a 2-terminal Joule heating approach.
- FIG. 68 Schematic of the IR-based TcEP process: (a) aligned SWNTs are grown on quartz, (b) a Tc resist is deposited on the SWNTs, (c) IR radiation is applied to induce thermocapillary flow, (d) reactive ion etching removes exposed SWNTs, (e) the Tc is removed.
- FIG. 69 Variation of laser power and rastering time for optimization of exposure dose and selectivity.
- AFM images of a substrate before processing (top) and after Tc resist deposition and exposure (middle) correspond perfectly with its Raman spectra (bottom).
- (f) I-V measurements conducted before and after exposure for a representative device. Histograms of I DS at V DS 2 V among 22 devices (g) before and (h) after exposure.
- FIG. 70 (a) An optical microscopy image of a thin film transistor integrating purified s-SWNTs after IR-based TcEP processing. Transfer (b) and output (c) characteristics of a representative device. (d) On/off ratio as a function of V D for the device.
- FIG. 71 Process and outcomes of the microwave-based purification of large-area arrays of aligned SWNTs.
- a second patterning defines the microstrip antennas (typically Ti) all over the substrate which serve as means to selectively transfer the microwave radiation energy into m-SWNTs. The resulting heating induces the local flow of a thermo-evaporated thin layer ( ⁇ 40 nm) of thermocapillary resist (Tc-resist) and opens trenches above the m-SWNTs.
- Tc-resist thermocapillary resist
- Inset represents the schematics of the geometry for extracting the trench depth, with key parameters defined.
- the SWNT, the Tc-resist and the substrate are grey, green and blue, respectively.
- (d) (e) Transfer curve and output characteristics for 40 transistors built with large-areas ( ⁇ 40 mm in total width) of SWNTs after the purification.
- the transistors utilize the Ti microstrip antenna as the source and drain contacts, and achieve an on/off ratio ⁇ 10 3 , and large output current-25 mA.
- FIG. 72 The effectiveness of the microwave-based purification process.
- FIG. 73 Coupling between the microwave field, microstrip antenna and SWNTs, and the heating mechanism.
- the SWNT is assumed to be a metallic type, with a resistivity of r s and inductance of L k per unit length.
- L c Within the contact length L c , strong coupling exist between the antenna and the SWNT, through a series of capacitance C g (geometric capacitance) and C q (quantum capacitance), and a shunt conductance G.
- C g geometric capacitance
- C q quantum capacitance
- the plot below shows the trench depth profile along the length of the SWNT extracted from the AFM image (black square) and the heating profile calculated from the circuit model (red curve).
- the heating as well as the trench depth is relatively constant along the SWNT.
- (e) Schematic illustration and AFM topography image of the case where the SWNT is in contact with one side of the antenna arms. The extracted trench depth (black square) decreases starting from the left contact, all the way to the end of the SWNT, where the trench eventually diminishes. This trend can be well captured by the FEM simulation of the heating profile (red curve), by using the resistivity of the SWNT as the only fitting parameter.
- FIG. 74 Different metals as the antennas.
- both the m-SWNTs and s-SWNTs create trenches (h>0); while for metals with low work-function like Mo, Ti, Mg, a distinct gap in the statistics separate the m-SWNTs from the s-SWNTs.
- the Al antenna results in a continuous distribution of the trench depth, and the gap appears when the thickness of the film decreases to ⁇ 20 nm. This suggests insufficient heating for the m-SWNTs, which is probably due to the poor wettability of Al to the SWNTs.
- the simulations match with the experimental results quite well, verifying that the Schottky barrier plays the key role for selectively heating the m-SWNT by using metals with low work-function as the antennas.
- FIG. 75 Microwave purification based on the removable antennas.
- FIG. 76 shows the schematic illustration of this process.
- the SWNTs are first grown on the quartz substrate, then followed by patterning strips of metal contacts on the SWNTs. Metals with relatively low work-function respect to the middle gap energy of the SWNTs, as well as good wettability to the SWNTs are typically good choices for the contacts.
- T c resist thermocapillary resist
- a DC voltage is applied across the metal contacts, which selectively induces the Joule heating on the m-SWNTs, causing the flows of the T c resist to open trenches above the m-SWNTs.
- FIG. 76 b shows the SEM image of a pristine array of SWNTs and the corresponding AFM topography image of the induced trenches. Such well-defined, uniform trenches along the SWNTs are the key for the successful removal.
- Statistics of the trench depth associated with each SWNT in a device with total 171 SWNTs are shown in FIG. 76 c . The depths are arranged in an ascending manner (x axis is the accumulated differential fraction of the SWNTs), showing that ⁇ 37% of the SWNTs creates trenches deep enough to etch away. This statistics matches that ⁇ 1 ⁇ 3 of the SWNTs are metallic types.
- FIG. 77 Pulsed biases for forming trenches in large and/or high density arrays of SWNTs.
- V GS +20V DC, 60 sec total stress duration, 50° C. background heating.
- FIG. 78 To qualitatively explain the selectivity of the 2T probe method, FIG. 78 a draws out the work-function of different types of metals, respect to the valence and conduction band of the SWNT. S-SWNTs are usually found to be P type, therefore metals with low work-function can form Schottky barriers to them, which will block the current flow ( FIG. 78 c ). On the other hand, the metals with relative high work-function can form Ohmic contact with the s-SWNTs, which enables the current flow ( FIG. 78 b ).
- FIG. 79 (a) SEM image of an individual metallic SWNT with a pair of electrode contacts of Ti/Pd on a quartz (Qz) substrate, (b) schematic illustration showing the SWNT (grey), the Qz substrate (blue), and a layer of PS (gold) after nanoscale thermocapillary flow induced by Joule heating in the SWNT.
- the parameter W Tc defines the width of the trench that forms.
- FIG. 81 (a) Trench width (W Tc ) and zero-shear viscosity ( ⁇ ) of polystyrene as a function of substrate temperature (T 0 ) between 313 K to 393 K, for PS films with different Mw (2.5, 9 and 30 kg/mol).
- the solid symbols and dashed lines correspond to measured and computed values for W Tc .
- the open symbols correspond to values of ⁇ computed using the Vogel equation.
- W Tc as a function of power per unit length dissipated in the SWNT (Q 0 ) from 8.4 ⁇ W/ ⁇ m to 214 ⁇ W/ ⁇ m for PS films with different M w (2.5, 5, 9 and 30 kg/mol).
- the solid symbols and dashed lines correspond to measured and computed values for W Tc .
- Carbon nanotube and “nanotube” are used synonymously and refer to allotropes of carbon comprising one or more cylindrically configured graphene sheets.
- Carbon nanotubes include single walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs).
- SWNTs single walled carbon nanotubes
- MWNTs multiwalled carbon nanotubes
- Carbon nanotubes typically have small diameters ( ⁇ 1-10 nanometers) and large lengths (up to several microns), and therefore may exhibit length to diameter ratios ⁇ 10 2 to about 10 7 .
- the longitudinal dimension of a nanotube is its length and the cross sectional dimension of a nanotube is its diameter (or radius).
- Carbon nanotubes include semiconducting carbon nanotubes, metallic carbon nanotubes, semi-metallic carbon nanotubes and mixtures of these.
- the compositions and methods of the present invention include a mixture of semiconducting and metallic carbon nanotubes, for example, wherein the ratio of semiconducting nanotubes to metallic nanotubes is selected from a range of 9-0.5.
- the compositions and methods of the present invention include a mixture of semiconducting and metallic carbon nanotubes, for example, wherein the ratio of semiconducting nanotubes to metallic nanotubes is greater than or equal to 1, preferably for some application greater than or equal to 2.
- the compositions and methods of the present invention include a mixture of semiconducting and metallic carbon nanotubes, for example, wherein the extent of semiconducting nanotubes is enriched, for example using fractionation or other purification techniques.
- “Supported by a substrate” refers to a structure that is present at least partially on a substrate surface or present at least partially on one or more intermediate structures positioned between the structure and the substrate surface.
- the term “supported by a substrate” may also refer to structures partially or fully embedded in a substrate, structures partially or fully immobilized on a substrate surface via an encapsulating layer (e.g., polymer layer) and structures partially or fully laminated on a substrate surface.
- substantially aligned nanotubes have lengths extending in longitudinal directions that are aligned with respect to each other but not provided in an absolutely parallel configuration.
- substantially aligned nanotubes have a partially linear geometry wherein their lengths assume a configuration with deviations from absolute linearity greater than about 10%, and in some embodiments with deviations from absolute linearity greater than about 20%.
- parallel refers to a geometry in which the lengths of carbon nanotubes are equidistant from each other for at least a portion of the points along their respective lengths and have the same direction or curvature.
- compositions and methods of the invention include carbon nanotube networks comprising substantially aligned nanotubes having at least one nanotube crossing.
- compositions and methods of the invention include carbon nanotube networks comprising randomly oriented nanotubes having at least one nanotube crossing.
- “Monolayer of nanotubes” refers to a layer of nanotubes on a substrate surface wherein the coverage of the area of the surface of the substrate having nanotubes is less than 100%, preferably for some embodiments substantially less than 100%.
- a monolayer refers to a layer of nanotubes wherein the coverage of the area of the surface of the substrate having nanotubes is less than 10%, preferably for some applications less than 2%, and preferably for some applications less than 1%.
- a monolayer refers to a layer of nanotubes wherein the coverage of the area of the surface of the substrate having nanotubes is selected over the range of 0.1-10%, or preferably for some embodiments selected over the range of 0.5-2%.
- a monolayer of carbon nanotubes has a thickness less than or equal to 20 nanometers, preferably for some applications less than or equal to 10 nanometers and preferably for some applications less than or equal to 5 nanometers.
- Use of monolayers of carbon nanotubes in some embodiments of the invention are useful for achieving effective gate modulation in a nanotube-based electronic devices.
- Flexible refers to a property of an object, such as a substrate, which is deformable in a reversible manner such that the object or material does not undergo damage when deformed, such as damage characteristic of fracturing, breaking, or inelastically deforming. Flexible polymers are useful with the methods described herein.
- flexible polymers include, but are not limited to: rubber (including natural rubber, styrene-butadiene, polybutadiene, neoprene, ethylene-propylene, butyl, nitrile, silicones), acrylic, nylon, polycarbonate, polyester, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyolefin, elastomers and other flexible polymers known to those of skill in the art.
- flexible objects or materials can undergo strain levels selected over the range of 1% to 1300%, 10% to 1300%, or 100% to 1300% without resulting in mechanical failure (e.g., breaking, fracturing or inelastically deforming).
- flexible objects or materials can be deformed to a radius of curvature selected over the range of 100 ⁇ m to 3 m without resulting in mechanical failure (e.g., breaking, fracturing or inelastically deforming).
- a “Schottky barrier” is a potential energy barrier for electrons formed at a metal-semiconductor junction. In some cases, thermally excited electrons may absorb sufficient energy to overcome the barrier.
- thermocapillary resist is an material capable of responding to a localized temperature increase by migrating away from a heat source. This occurs when the material has a higher affinity for itself than for the surface material of the heated object.
- a “thermocapillary resist” also protects objects against at least one chemical and/or physical challenge applied to the resist.
- a thermocapillary resist may comprise an encapsulating material, a coating material and/or a conformal material.
- substantially uniform refers to a continuous, pinhole free layer of material.
- Nanotube crossings in this context refers to a configuration wherein two or more nanotubes are in electrical contact, physical contact and/or in an overlapping configuration.
- nanotube crossings in some embodiments refers to a configuration with two, three or four different nanotubes are provided on top of or underneath each other.
- Electrode generally refers to a device incorporating a plurality of components, and includes large area electronics, printed wire boards, integrated circuits, component arrays, biological and/or chemical sensors, physical sensors (e.g., temperature, strain, etc.), nanoelectromechanical systems, microelectromechanical systems, photovoltaic devices, communication systems, medical devices, optical devices and electro-optic devices.
- semiconductor refers to any material that is an insulator at a very low temperature, but which has an appreciable electrical conductivity at a temperature of about 300 Kelvin. In the present description, use of the term semiconductor is intended to be consistent with use of this term in the art of microelectronics and electronic devices.
- a “component” is used broadly to refer to an individual part of a device.
- Components include, but are not limited to, thin film transistors (TFTs), transistors, diodes, electrodes, integrated circuits, circuit elements, control elements, photovoltaic elements, photovoltaic elements (e.g. solar cell), sensors, light emitting elements, actuators, piezoelectric elements, receivers, transmitters, microprocessors, transducers, islands, bridges and combinations thereof.
- Components may be connected to other components such as one or more contact pads as known in the art, such as by metal evaporation, wire bonding, and application of solids or conductive pastes, for example.
- Electronic devices of the invention may comprise one or more components, optionally provided in an interconnected configuration.
- Encapsulate refers to the orientation of one structure such that it is at least partially, and in some cases completely, surrounded by one or more other structures, such as a substrate, adhesive layer or encapsulating layer. “Partially encapsulated” refers to the orientation of one structure such that it is partially surrounded by one or more other structures, for example, wherein 30%, or optionally 50% or optionally 90%, of the external surfaces of the structure are surrounded by one or more structures. “Completely encapsulated” refers to the orientation of one structure such that it is completely surrounded by one or more other structures.
- Contiguous refers to materials or layers that are touching or connected throughout in an unbroken sequence.
- a contiguous layer of an implantable biomedical device has not been etched to remove a substantial portion (e.g., 10% or more) of the originally provided material or layer.
- Substrate refers to a material, layer or other structure having a surface, such as a receiving surface, that is capable of supporting one or more components or devices.
- a component that is “bonded” to the substrate refers to a component that is in physical contact with the substrate and unable to substantially move relative to the substrate surface to which it is bonded.
- Unbonded components or portions of a component are capable of substantial movement relative to the substrate and may be referred to herein as in physical contact with the substrate.
- Processing is used broadly to refer to treatment of a device or surface to obtain one or more desired functional attributes, including a functional attribute such as electrical interconnection.
- processing include providing a material such as by deposition of one or more components, including active electronic components, structural, barrier, or encapsulating layer(s), removal or partial removal of materials, or transformation or partial transformation of materials to obtain a desired physical parameter.
- Dielectric refers to a non-conducting or insulating material.
- an inorganic dielectric comprises a dielectric material substantially free of carbon.
- Specific examples of inorganic dielectric materials include, but are not limited to, silicon nitride, silicon dioxide, silk, silk composite, elastomers and polymers.
- Polymer refers to a macromolecule composed of repeating structural units connected by covalent chemical bonds or the polymerization product of one or more monomers, often characterized by a high molecular weight.
- the term polymer includes homopolymers, or polymers consisting essentially of a single repeating monomer subunit.
- the term polymer also includes copolymers, or polymers consisting essentially of two or more monomer subunits, such as random, block, alternating, segmented, grafted, tapered and other copolymers.
- Useful polymers include organic polymers or inorganic polymers that may be in amorphous, semi-amorphous, crystalline or partially crystalline states. Crosslinked polymers having linked monomer chains are particularly useful for some applications.
- Polymers useable in the methods, devices and components include, but are not limited to, plastics, elastomers, thermoplastic elastomers, elastoplastics, thermoplastics and acrylates.
- Exemplary polymers include, but are not limited to, acetal polymers, biodegradable polymers, cellulosic polymers, fluoropolymers, nylons, polyacrylonitrile polymers, polyamide-imide polymers, polyimides, polyarylates, polybenzimidazole, polybutylene, polycarbonate, polyesters, polyetherimide, polyethylene, polyethylene copolymers and modified polyethylenes, polyketones, poly(methyl methacrylate), polymethylpentene, polyphenylene oxides and polyphenylene sulfides, polyphthalamide, polypropylene, polyurethanes, styrenic resins, sulfone-based resins, vinyl-based resins, rubber (including natural rubber, styrene-butadiene,
- Elastomeric stamp and “elastomeric transfer device” are used interchangeably and refer to an elastomeric material having a surface that can receive as well as transfer a material.
- Exemplary conformal transfer devices useful in some methods of the invention include elastomeric transfer devices such as elastomeric stamps, molds and masks. The transfer device affects and/or facilitates material transfer from a donor material to a receiver material.
- a method of the invention uses a conformal transfer device, such as an elastomeric transfer device (e.g.
- stamp in a microtransfer printing process, for example, to transfer one or more single crystalline inorganic semiconductor structures, one or more dielectric structures and/or one or more metallic conductor structures from a fabrication substrate to a device substrate.
- Stamp is used broadly herein to refer to a substrate that can pick up components from one surface and transfer them to another surface.
- the stamp may be a material having an adhesive surface that facilitates pick-up by adhesive forces, wherein the adhesive forces are less than subsequent contact forces when the stamp is brought into contact with a receiving surface.
- the transfer printing may be direct surface-to-surface contact, from donor to a receiving surface.
- Elastomer refers to a polymeric material which can be stretched or deformed and returned to its original shape without substantial permanent deformation. Elastomers commonly undergo substantially elastic deformations. Useful elastomers include those comprising polymers, copolymers, composite materials or mixtures of polymers and copolymers. Elastomeric layer refers to a layer comprising at least one elastomer. Elastomeric layers may also include dopants and other non-elastomeric materials.
- elastomers include, but are not limited to, thermoplastic elastomers, styrenic materials, olefinic materials, polyolefin, polyurethane thermoplastic elastomers, polyamides, synthetic rubbers, PDMS, polybutadiene, polyisobutylene, poly(styrene-butadiene-styrene), polyurethanes, polychloroprene and silicones.
- an elastomeric stamp comprises an elastomer.
- Exemplary elastomers include, but are not limited to silicon containing polymers such as polysiloxanes including poly(dimethyl siloxane) (i.e.
- PDMS and h-PDMS poly(methyl siloxane), partially alkylated poly(methyl siloxane), poly(alkyl methyl siloxane) and poly(phenyl methyl siloxane), silicon modified elastomers, thermoplastic elastomers, styrenic materials, olefinic materials, polyolefin, polyurethane thermoplastic elastomers, polyamides, synthetic rubbers, polyisobutylene, poly(styrene-butadiene-styrene), polyurethanes, polychloroprene and silicones.
- a polymer is an elastomer.
- Conformable refers to a device, material or substrate which has a bending stiffness that is sufficiently low to allow the device, material or substrate to adopt any desired contour profile, for example a contour profile allowing for conformal contact with a surface having a pattern of relief features.
- a desired contour profile is that of a tissue in a biological environment.
- Conformal contact refers to contact established between a device and a receiving surface.
- conformal contact involves a macroscopic adaptation of one or more surfaces (e.g., contact surfaces) of a device to the overall shape of a surface.
- conformal contact involves a microscopic adaptation of one or more surfaces (e.g., contact surfaces) of a device to a surface resulting in an intimate contact substantially free of voids.
- conformal contact involves adaptation of a contact surface(s) of the device to a receiving surface(s) such that intimate contact is achieved, for example, wherein less than 20% of the surface area of a contact surface of the device does not physically contact the receiving surface, or optionally less than 10% of a contact surface of the device does not physically contact the receiving surface, or optionally less than 5% of a contact surface of the device does not physically contact the receiving surface.
- a method of the invention comprises establishing conformal contact between a conformal transfer device and one or more single crystalline inorganic semiconductor structures, one or more dielectric structures and/or one or more metallic conductor structures, for example, in a microtransfer printing process, such as dry transfer contact printing.
- the receiving surface may functionally correspond to a release layer that is later used to release the components from a substrate that supports the release layer.
- Young's modulus is a mechanical property of a material, device or layer which refers to the ratio of stress to strain for a given substance. Young's modulus may be provided by the expression:
- E Young's modulus
- L 0 the equilibrium length
- ⁇ L the length change under the applied stress
- F the force applied
- A the area over which the force is applied.
- Young's modulus may also be expressed in terms of Lame constants via the equation:
- High Young's modulus (or “high modulus”) and low Young's modulus (or “low modulus”) are relative descriptors of the magnitude of Young's modulus in a given material, layer or device.
- a high Young's modulus is larger than a low Young's modulus, preferably about 10 times larger for some applications, more preferably about 100 times larger for other applications, and even more preferably about 1000 times larger for yet other applications.
- a low modulus layer has a Young's modulus less than 100 MPa, optionally less than 10 MPa, and optionally a Young's modulus selected from the range of 0.1 MPa to 50 MPa.
- a high modulus layer has a Young's modulus greater than 100 MPa, optionally greater than 10 GPa, and optionally a Young's modulus selected from the range of 1 GPa to 100 GPa.
- a device of the invention has one or more components, such as substrate, encapsulating layer, inorganic semiconductor structures, dielectric structures and/or metallic conductor structures, having a low Young's modulus.
- a device of the invention has an overall low Young's modulus.
- Low modulus refers to materials having a Young's modulus less than or equal to 10 MPa, less than or equal to 5 MPa or less than or equal to 1 MPa.
- Guided growth refers to growth of carbon nanotubes on a substrate wherein growth of individual nanotubes occurs along nanotube growth axes having selected spatial orientations, such as an orientation that is parallel to at least a portion of the growth axes of other nanotubes in the array and/or an orientation that is parallel to a principle guided growth axis of the guided growth substrate.
- Guided growth in the present invention arises from electrostatic, energetic and/or steric interactions between the nanotubes and or catalyst with the guided growth substrate.
- guided growth of nanotubes may occur via a mechanism involving energetically favorable van der Waals interactions between growing nanotubes and/or catalyst particles with the lattice arrangement of the guided growth substrate.
- Guided growth of nanotubes may also occur via interaction of the nanotubes and/or catalyst particles with step edges, microfacets, nanofacets or other surface features of the receiving surface of the guided growth substrate.
- Guided deposition refers to assembling and/or positioning materials, such as carbon nanotubes, on a substrate via a concerted process providing for spatial orientation, position and/or organization selected with good accuracy and precision.
- guided deposition methods of the present invention provide a means of assembling and/or positioning carbon nanotubes in spatial orientations and positions selected such that their longitudinal axes are parallel to a principle guided deposition axis of a guided deposition substrate.
- guided deposition methods of the present invention provide a means of assembling and/or positioning carbon nanotubes in orientations and positions wherein their longitudinal axes are parallel to each other.
- “Parallel to a principle guided growth axis” refers to a spatial configuration of one or more carbon nanotubes wherein the length of the carbon nanotube is substantially equidistant to the principle guided growth axis of a guided growth substrate for at least some points along the length of the nanotube. “Parallel to a principle guided deposition axis” refers to a spatial configuration of one or more carbon nanotubes wherein the length of the carbon nanotube is substantially equidistant to the principle guided deposition axis of a guided deposition substrate for at least some points along the length of the nanotube. As used in this context, the term parallel is intended to encompass some deviation from absolute parallelism.
- Nanotubes parallel to a principle guided growth or deposition axis may have parallel spatial orientations with deviations from absolute parallelism that are less than or equal to 20 degrees, preferably deviations from absolute parallelism that are less than or equal to 10 degrees for some applications, and preferably deviations from absolute parallelism that are less than 1 degrees for some applications.
- the present invention provides nanotube arrays and related methods of making nanotube arrays wherein at least 95% of the nanotubes in the array extend lengths that are parallel to each other and/or parallel to a principle guided growth or deposition axis with deviations from absolute parallelism of less than or equal to 20 degrees.
- FIG. 1A provides a schematic flow diagram illustrating a method of the present invention for purifying carbon nanotubes, for example, by separation of a first subset of nanotubes having one or more selected physical or chemical properties from other nanotubes in a nanotube precursor layer.
- aspects of this method are particularly useful for purifying carbon nanotubes on the basis of optical and/or electronic properties, such as optical absorption, electronic conductivity, interaction with an electromagnetic field, electric field, magnetic field, etc.
- the aspect of the invention provides a means of selectively removing or transferring metallic nanotubes from a precursor layer comprising a mixture of longitudinally aligned metallic and semiconducting carbon nanotubes.
- aspects of the methods of the present invention are particularly well suited for purification of SWNTs, including layers and thin films containing SWNTs such as monolayer and submonolayer SWNT layers.
- a precursor layer of substantially aligned carbon nanotubes comprising a mixture of first carbon nanotubes (e.g., a first subset of nanotubes having one or more selected physical or chemical property) and second carbon nanotubes (e.g., a second subset different from the first subset), for example, providing as a layer or film of substantially aligned carbon nanotubes including both metallic and semiconducting carbon nanotubes.
- first carbon nanotubes e.g., a first subset of nanotubes having one or more selected physical or chemical property
- second carbon nanotubes e.g., a second subset different from the first subset
- the precursor layer is covered with a thermocapillary resist provided in thermal contact, and optionally in physical contact, with the carbon nanotubes, for example, in thermal contact with at least a portion of, and optionally all of, the first carbon nanotubes.
- the process further includes selectively heating the first carbon nanotubes to generate a thermocapillary flow of the thermocapillary resist away from the first carbon nanotubes, thereby exposing at least a portion of the first carbon nanotubes of the precursor layer.
- metallic nanotubes are selectively heated in a manner such that semiconducting nanotubes in the precursor layer do not undergo an equivalent increase in temperature.
- the process further includes separating at least a portion of the first carbon nanotubes, such as the exposed first nanotubes, from the second carbon nanotubes to generate a purified layer of carbon nanotubes.
- the exposed first nanotubes are separated via removal from the precursor layer, for example via chemical degradation or transfer processing.
- FIG. 1B provides a schematic flow diagram corresponding to a method of the present invention further illustrating various process approaches for selective heating the first carbon nanotubes having one or more selected physical or chemical properties.
- selective heating is achieved in some embodiments via absorption of electromagnetic radiation (EMR), for example, via absorption of continuous or pulsed EMR from a laser source or a microwave source.
- EMR electromagnetic radiation
- selective heating is achieved in some embodiments via applying a voltage and/or electricfield so as to selectively generate current in the first carbon nanotubes having one or more selected physical or chemical properties.
- a transistor or direct two terminal device structure is used for applying an electromagnetic field or voltage to the carbon nanotubes undergoing processing, thereby selectively generating an increase in temperature for the first carbon nanotubes relative to the second carbon nanotubes.
- a voltage applied across the nanotubes and an electric field is simultaneously applied, for example, using a gate electrode positioned proximate to carbon nanotubes extending from source and drain electrodes.
- FIG. 1C provides a schematic flow diagram corresponding to a method of the present invention further illustrating various process approaches for selective removal of the first carbon nanotubes that undergo selective heating.
- exposure of the first nanotubes allows access for subsequent removal via a range of processes.
- exposed carbon nanotubes are removed in some embodiments by chemical degradation processing that optionally destroy the carbon nanotubes, for example, via etching, exposure to a plasma, chemical removal or ablation.
- selective removal is achieved in some embodiments via transfer processing that optionally does not result in degradation of the first carbon nanotubes, for example, via transfer printing (e.g., dry transfer printing) or via solution processing, such as by contacting the exposed nanotubes with a solution, dissolution and subsequent transfer away from the precursor layer.
- transfer is achieved via transfer printing via contacting the exposed first carbon nanotubes with an elastomeric stamp.
- FIG. 1D provides a schematic flow diagram corresponding to a method of the present invention further illustrating various process approaches for generating a precursor layer comprising substantially aligned carbon nanotubes.
- a guide growth substrate is provided in some embodiments, and substantially longitudinally aligned carbon nanotubes are grown on a receiving surface of the guided growth substrate.
- the precursor layer comprising substantially longitudinally aligned carbon nanotubes is achieved in some embodiments via contacting the receiving surface of a guided deposition substrate with a solution containing suspended carbon nanotubes, which optionally undergo self assembly to form the precursor layer.
- SWNTs single walled carbon nanotubes
- the most advanced opportunities demand the ability to form perfectly aligned, horizontal arrays of purely semiconducting, chemically pristine SWNTs.
- nanoscale thermocapillary flows in thin film organic coatings serve as highly efficient means for selectively removing metallic SWNTs from electronically heterogeneous aligned arrays grown on quartz substrates.
- the low temperatures and unusual physics associated with this process enable robust, scalable operation, with clear potential for practical use.
- Detailed experimental and theoretical studies reveal all of the essential attributes of the underlying thermophysical phenomena.
- Demonstrations include use of purified arrays in transistors with mobilities and on/off switching ratios that can exceed ⁇ 1000 cm 2 /Vs and ⁇ 10,000, respectively, and with current outputs in the mA range; simple logic gates built using such devices represent first steps toward integration into circuits.
- the disadvantages are that the resulting SWNTs are typically short ( ⁇ 1 ⁇ m), chemically modified and/or coated, and difficult to assemble into arrays with high degrees of alignment 15,17-19 .
- the second approach overcomes these limitations through the use of chemical vapor deposition techniques that, when used with quartz substrates, can yield nearly perfectly linear (>99.9% of SWNTs within 0.01° of perfect alignment), aligned arrays of long (100 ⁇ m and up to ⁇ millimeters) and chemically pristine SWNTs 3,20-23 .
- the main difficulty is in removing the m-SWNTs from such arrays.
- the required high power operation ( ⁇ 90 ⁇ W/ ⁇ m for channel lengths >1 ⁇ m, increasing as channel length decreases to values >1 mW/ ⁇ m) 29-31 leads to shifts in threshold voltage, avalanche effects 32 , band-to-band tunneling, failure in gate dielectrics, and significant heat sinking at the contacts 29 , all of which can prevent proper operation of the process. More significantly, breakdown only removes the m-SWNTs in isolated, narrow regions ( ⁇ 100 nm lengths), with positions that are not well controlled 34 . As a result, the vast majority of the m-SWNTs remain in the arrays, thereby preventing generalized use in subsequently fabricated devices.
- FIG. 2( i ) a,b shows schematic illustrations and corresponding atomic force microscope images of the process applied to a heterogeneous collection of SWNTs grown on quartz.
- Arrays formed in this fashion consist of individual, isolated SWNTs, with very few multi-walled nanotubes or bundles of SWNTs, but with a distribution of diameters between ⁇ 0.6 and ⁇ 2.0 nm and a range of chiralities 23,31 .
- the key element in the purification process is an ultrathin ( ⁇ 25 nm) amorphous layer of a small molecule organic species, in this example ⁇ , ⁇ , ⁇ ′-Tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene 36 , deposited uniformly over the arrays of SWNTs by thermal evaporation.
- this film FIG. 8
- thermocapillary resist a thermocapillary resist.
- the particular material used here is well suited for present purposes because it combines hydroxyl and phenyl moieties to facilitate formation of uniform, continuous coatings on the surfaces of both the quartz and the SWNTs. This behavior is critical for its role as an effective etch resist at extremely small thicknesses.
- Metal and dielectric layers patterned at the edges of an area of interest enable current injection primarily into only the m-SWNTs, due to controlled electrostatically induced increases in the heights and widths of the Schottky barriers at the source ends of s-SWNTs ( FIGS. 1 a , 10 and 31 - 33 ). These layers represent removable, transistor structures in which the gates extend beyond the source electrodes by a distance small ( ⁇ 5 ⁇ m) compared to the separation between the source and drain ( ⁇ 30 ⁇ m).
- thermocapillary resist ( FIG. 1 b ).
- inert environments can also be used (e.g. dry nitrogen, or argon). Excluding oxygen can help to prevent electrical breakdown in extreme cases of hot spots along the lengths of the SWNTs with localized defects.
- Reactive ion etching (O 2 /CF 4 ) after thermocapillary flow eliminates only the m-SWNTs. Removing the residual thermocapillary resist and the metal/dielectric structures leaves a purified array of s-SWNTs, in a configuration well suited for planar integration into diverse classes of devices and sensors.
- SWNT metallic (semiconducting) if the ratio between the on (I on ) and off (I off ) currents (i.e. on/off ratio) in a well-designed transistor structure that incorporates this SWNT is less than (greater than) ⁇ 100. This definition places SWNTs that are sometimes referred to as quasi-metallic into the m-SWNT classification.
- 2( i ) c shows a representative transfer characteristic for a device before and after purification, measured in air using the same metal/dielectric structures that enable selective Joule heating.
- the results illustrate a dramatic reduction in I off (from 0.7 ⁇ A to 2 ⁇ 10 ⁇ 5 ⁇ A), thereby improving the on/off ratio from 2.7 to 3 ⁇ 10 4 .
- the relatively small numbers ( ⁇ 30) of SWNTs in each device used to examine the statistics lead to the conclusion that the observed on/off ratios correspond to complete removal (i.e. 100%) of the m-SWNTs.
- FIG. 2 ( ii ) b shows topographical images of the same array coated with thermocapillary resist after application of several increasing values of V DS (direct current for 5 min; substrate temperature 60° C.). A key observation is that the distributions in expansion, and therefore temperature ( FIG.
- thermocapillary resist correspond directly to the geometries of trenches that appear in the thermocapillary resist ( FIG. 2 b ).
- trenches progressively form with increasing V DS in an order consistent with the temperatures revealed by scanning Joule expansion microscopy, e.g. trenches at the second and fifth SWNT from the right appear first and last, respectively.
- Related effects can be observed along an individual SWNT, where trenches nucleate in areas of enhanced temperature (‘hot spots’; arrows in FIG. 2 a and the top frame of FIG. 2 b ).
- FIG. 2 ( ii ) c shows a representative cross-sectional profile of E 0 for the case of a SWNT with length ⁇ 3.5 ⁇ m, along with the corresponding scanning Joule expansion microscopy image ( FIG. 11 ).
- the power density per unit length is Q(t) with Q 0 estimated to be ⁇ 13 ⁇ W/ ⁇ m based on the total input power into the device, which includes three SWNTs on an SiO 2 /Si substrate.
- ⁇ ⁇ ( x , y ) 1 2 ⁇ k s ⁇ ⁇ ⁇ ⁇ - L / 2 L / 2 ⁇ d ⁇ ⁇ ⁇ 0 ⁇ ⁇ Q 0 ⁇ J 0 ⁇ ( ⁇ ⁇ ( ⁇ - y ) 2 + x 2 ) cosh ⁇ ( ⁇ ⁇ ⁇ h f ) + k f k s ⁇ sinh ⁇ ( ⁇ ⁇ ⁇ h f ) ⁇ d ⁇ ( 1 )
- k s and k f are the thermal conductivity of the thermocapillary resist and quartz substrate, respectively
- h f is the thickness of the resist.
- FIG. 2 ( ii ) e presents results for the second SWNT from the right, extracted from FIG. 2 a and the bottom frame of FIG. 2 b .
- FIG. 2 ( ii ) f shows the average W Tc for a number of different, individual SWNTs as a function of Q 0 . Similar values occur over ranges of power ( ⁇ 10-40 ⁇ W/ ⁇ m) that exceed those associated with optimized conditions for purification. This behavior is much different than that expected from other thermally driven processes, such as sublimation or ablation, which typically involve abrupt temperature thresholds ( FIG. 19 ).
- thermocapillarity in systems where the dimension along the SWNTs can be considered infinite corresponds to unidirectional flow in which the thickness profile in the thermocapillary resist can be written h(x,t) with
- FIG. 3 f shows the measured time dependence of W Tc for two SWNTs, where both roughly follow the expected theoretical behavior, namely W Tc ⁇ t 0.25 (see FIGS. 13-15 ).
- thermocapillary resists include large temperature coefficients of surface tension and low viscosities. Furthermore, decreasing the thickness reduces the trench widths. Empirical studies of various materials for thermocapillary resists (see Supplementary Information, FIG. 18 ) led to the selection of the molecular glass reported here.
- thermocapillary enabled purification is in a preparatory mode, where it serves as one of the several steps, such as substrate cleaning, SWNT growth, transfer and others that occur before device processing.
- Such a scheme decouples purification from any detailed consideration in component or circuit layout, and is made possible by the ability to eliminate entirely all of the m-SWNTs.
- Two approaches can be considered. In the first, one or a small number of electrode structures, each with large lateral extent as illustrated in FIGS. 4 a - b , enable elimination of m-SWNTs over significant areas.
- processing occurs on hundreds or thousands of SWNTs at once, using pulsed currents to avoid cumulative heating (see Supplementary Information, FIG. 17 ).
- FIG. 4 c shows the electrical characteristics of the structure in FIG.
- FIG. 4 d illustrates an alternative approach, in which smaller pairs of interconnected electrodes provide for purification in distributed regions, capable of lithographic alignment at a coarse level to areas of interest in a final application. Effects on I on and I off in this case are in the range of those achieved in other geometries (see Supplementary Information, FIG. 21 , Table 2).
- densities 20 and area averaged ( FIG. 17 ) densities of a few per micron. Improved densities can be realized using transfer techniques. 42-44 Although high densities can be important in electronics, modest or low densities can be useful in sensors and other devices.
- a re-usable bottom electrode structure can be exploited to eliminate cycles of processing that would otherwise be necessary for repetitive fabrication of top electrode structures described previously.
- a single, re-usable substrate provides a fully formed, bottom split gate structure for use in the purification process. Aligned arrays of SWNTs transferred to this substrate using techniques described previously 42-44 can be processed to remove m-SWNTs. The remaining s-SWNTs can then be transferred to a final device substrate.
- FIG. 5 a schematically illustrates two cycles of this process.
- FIG. 5 b,c present transfer characteristics of arrays of SWNTs before and after purification, performed with a single back gate structure in two separate cycles of use. Additional details appear in FIGS. 25 and 26 .
- FIG. 6 a,b Devices with short channel lengths ( ⁇ 800 nm) defined using a near-field phase shift lithography technique 50 provide a demonstration, as shown in FIG. 6 a,b .
- FIG. 6 c presents electrical properties that are consistent with those of long channel devices when effects of contact resistance are included. The observed hysteresis has known origins that can be minimized using strategies described elsewhere 45-48 .
- a simple logic gate, consisting of two transistors using arrays of s-SWNTs, provides an additional example of the utility of this process, as illustrated in FIG. 6 d,e .
- FIG. 6 f shows the voltage transfer characteristics and gain associated with this p-type inverter. The peak gain is ⁇ 4, consistent with expectation for this design (see Supplementary Information, FIGS. 22-23 ).
- the purification method introduced here provides scalable and efficient means for converting heterogeneous arrays of SWNTs into those with purely semiconducting character.
- An important advantage is that the processing steps are fully compatible with fabrication tools used for commercial manufacture of digital electronics and display backplanes.
- Reactive ion etching 100 mTorr, 20 sccm O 2 , 100 W, 30 s; Plasma-Therm
- SWNTs everywhere except for regions between these electrodes.
- Prebaking 250° C., 2 hr, in a glove box
- SOG spin-on glass
- Atomic layer deposition (80° C.; Cambridge NanoTech) created films of Al 2 O 3 (30 nm) on top of the SOG.
- Photolithography (AZ 5214) and etching (6:1 BOE for 50 s) removed the SOG/Al 2 O 3 bilayer from the region between the source/drain electrodes.
- Prebaking (110° C., 10 min) a spin cast (4000 rpm, 60 s) solution of polyvinyl alcohol (PVA; M w between 89,000 and 98,000, 99%, hydrolyzed, Sigma-Aldrich; solvent: D.I.
- PVA polyvinyl alcohol
- thermocapillary resist ⁇ , ⁇ , ⁇ ′-Tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene; TCI international.
- V DS ⁇ 40 to ⁇ 50 V, corresponding to fields of V DS /L ch ⁇ 1.33-1.66 V/ ⁇ m
- biasing the source/gate to +20 V under vacuum ( ⁇ 10 ⁇ 4 Torr, Lakeshore) and holding the substrate temperature at 60° C., all for ⁇ 5 min, yielded trenches in the thermocapillary resist at the locations of the m-SWNTs.
- Reactive ion etching (10 mTorr, 1 sccm O 2 , 1 sccm CF 4 , 75 W, 20 s; Plasma-Therm RIE) eliminated the m-SWNTs exposed in this manner, without affecting the s-SWNTs.
- Immersion in acetone for 30 min removed the thermocapillary resist, to complete the process.
- Electrodes were wire bonded to a sample holder (Spectrum Semiconductor Materials) to allow contact mode atomic force microscopy (Asylum MFP 3D and Cantilever Asylum # Olympus AC240TS) while applying suitable biases to the electrode structures.
- the bias consisted of a sinusoidal voltage with amplitude of 5 V and frequency of 386 kHz. Measurements on SiO 2 (200 nm)/Si, used similar two terminal devices, but with spin cast overcoats of poly(methylmethacrylate) (Microchem. 950 A2) with thicknesses of ⁇ 120 nm.
- the bias in such cases consisted of a sinusoidal voltage with amplitude of 3V and frequency of 30 kHz with the substrate electrically grounded.
- thermocapillary resist ⁇ 25 nm
- atomic force microscope Anasylum research ORCA sample mount
- Images collected by fast scanning ⁇ 30 s acquisition times) defined the topography of a small region of interest.
- application of electrical biases for durations, of 0.1 s at short times and increasing to 30 min at long times caused the trenches to increase in width by controlled amounts.
- Photolithography and etching defined gate electrodes (2 nm Cr and 13 nm Pd).
- Source and drain electrodes (2 nm Cr and 13 nm Pd) were formed using the same procedures as those for the gate completed the fabrication.
- the 3D finite element model for the temperature distributions used eight-node, hexahedral brick elements in a finite element software package (ABAQUS) to discretize the geometry.
- the SWNT was treated as a volume heat source, with a zero heat flux boundary at the top surface of Tc-resist, and a constant temperature T ⁇ the bottom of the quartz substrate.
- the equations of motion represent a pair of coupled partial differential equations
- thermocapillary resists Tc-resists
- Tc-resists thermocapillary resists
- devices were fabricated in geometries to ensure that only 1 or 2 SWNTs were present in the channel, as confirmed by AFM.
- the electronic type could be determined directly from the electrical properties (on/off ratio >100, s-SWNT; on/off ratio ⁇ 100, m-SWNT).
- both SWNTs must be s-SWNTs.
- one SWNT must be a m-SWNT and the other a s-SWNT.
- Devices with two m-SWNTs were not used for these experiments. Biases were applied to increase the resistance of the s-SWNTs (i.e. their “off” state, at +20 V GS ), resulting in relatively low (high) current levels for all s-SWNTs (m-SWNTs). As a result, all 2 SWNT devices fell into one of two cases: their currents were dominated by a single m-SWNT or they contained two s-SWNTs.
- optimized conditions can be established. At fields below the optimal range, the heating is insufficient to yield trenches along the entire lengths of all of the m-SWNTs; at higher fields, the most conductive s-SWNTs begin to show partial trench formation. However, for optimized conditions, all s-SWNTs yielded no trenches, while all m-SWNTs yield complete trenches, as required for proper operation of TcEP.
- FIG. 8 a shows the chemical structure of the Tc-resist.
- the material was deposited via thermal evaporation.
- FIG. 8 b shows AFM images of a film deposited on a SiO 2 /Si substrate. The surface roughness is comparable to that of the underlying substrate, i.e. 2-3 ⁇ .
- heated substrates quartz or SiO 2 /Si
- thin coatings of Tc-resist ⁇ 25 nm thickness
- FIG. 9 a shows a histogram of the conductances of individual SWNTs determined in this way.
- the mean conductances for m-SWNTs and s-SWNTs are 17 k ⁇ / ⁇ m and 75 k ⁇ / ⁇ m, respectively. These distributions are in the range of those reported for single SWNTs studied previously 2 ; with values of 35 k ⁇ / ⁇ m (m-SWNTs) and 55 k ⁇ / ⁇ m (s-SWNTs) for backgated devices ( FIG. 9 c ) 2 ; and 140 k ⁇ / ⁇ m (m-SWNTs) and 1000 k ⁇ / ⁇ m (s-SWNTs) for top gated devices 2 .
- the assumption that TcEP preserves all s-SWNT is consistent with observations.
- the electrode geometries used for TcEP involved a partial gate structure shown schematically in FIG. 1 a .
- This configuration results in reduced gate-drain fields, which minimize Schottky barrier tunneling, band-to-band tunneling and avalanche phenomena 3 .
- the operation avoids ambipolar conduction at the bias conditions needed for TcEP (characterization at conditions consistent with TcEP, 60° C. background heating, ⁇ 1 ⁇ 10 4 torr).
- FIG. 9 c illustrates the effect of source-drain bias and gate overlap (L ov ) on on/off ratio, where all measurements were performed on the same SWNT.
- the device with 5 ⁇ m gate overlap i.e. the configuration used for TcEP
- exhibits on/off ratios 2-3 orders of magnitude higher than the device with full overlap (L ov 30 ⁇ m).
- FIG. 11 a shows the integrated expansion percentage (I.E.P.). To determine this quantity, we first located the maximum expansion along each measured cross-section perpendicular to the length of the SWNT. Next, we integrated these values along each SWNT, to get the integrated expansion (I.E.). The I.E.P. is the ratio of the I.E.
- FIG. 11 c shows a 3D rendering of the SJEM signal for the SWNT associated with FIG. 2 b .
- FIG. 11 d shows a schematic of the SJEM measurement.
- ⁇ 0 + ⁇ 0 - , - k 0 ⁇ ⁇ ⁇ ⁇ z ⁇
- the temperature rise due to the disk heat source can be obtained by Eq. (11).
- r 0 ⁇ 0 we obtain the temperature rise due to a point heat source as
- ⁇ ⁇ ( x , y ) 1 4 ⁇ ⁇ k 1 ⁇ ⁇ ⁇ ⁇ - L / 2 L / 2 ⁇ ⁇ d ⁇ ⁇ ⁇ 0 + ⁇ ⁇ ( ⁇ + 1 ) ⁇ J 0 ⁇ ( ⁇ ⁇ ( ⁇ - y ) 2 + x 2 ) ⁇ ⁇ ( 1 - ⁇ ) ⁇ cosh ⁇ ( h 0 ⁇ ⁇ 2 + q 0 2 ) + ( ⁇ + 1 ) ⁇ k 0 ⁇ ⁇ 2 + q 0 2 k 1 ⁇ ⁇ 2 + q 1 2 ⁇ sinh ⁇ ( h 0 ⁇ ⁇ 2 + q 0 2 ) ⁇ Q 0 ⁇ 2 + q 1 2 ⁇ ⁇ d ⁇ ( 16 )
- Eq. (16) gives the magnitude of time oscillating temperature rise, i.e., ⁇ 0 in the main text is equal to 2 ⁇ (x,y).
- boundary conditions involve continuous temperature and heat flow at all material interfaces except those with the SWNT, negligible heat flow at the top surface and a constant temperature at the base of the substrate.
- discontinuous heat flow is assumed, as a means to introduce the Joule heat source.
- ⁇ ⁇ ( x , y ) 1 2 ⁇ ⁇ k s ⁇ ⁇ ⁇ ⁇ - L / 2 L / 2 ⁇ ⁇ d ⁇ ⁇ ⁇ 0 ⁇ ⁇ Q 0 ⁇ J 0 ⁇ ( ⁇ ⁇ ( ⁇ - y ) 2 + x 2 ) cosh ⁇ ( ⁇ ⁇ ⁇ h f ) + k f k s ⁇ sinh ⁇ ( ⁇ ⁇ ⁇ h f ) ⁇ ⁇ d ⁇ ( 18 )
- k f and k s are the thermal conductivity of Tc-resist and quartz, respectively and h f is the thickness of Tc-resist.
- a 3D finite element model was established to study the temperature distribution in the system and validate the analytical model.
- Eight-node, hexahedral brick elements in the finite element software ABAQUS are used to discretize the geometry.
- a volume heat source was applied on the SWNT.
- the zero heat flux boundary was applied at the top surface of the Tc-resist, and a constant temperature T ⁇ is applied at the bottom of the quartz substrate.
- ⁇ ⁇ ( x , y ) 1 k f ⁇ ⁇ ⁇ ⁇ - L / 2 L / 2 ⁇ ⁇ d ⁇ ⁇ ⁇ 0 ⁇ ⁇ Q 0 ⁇ e - ⁇ ⁇ ⁇ h f ⁇ ( 1 + k s ⁇ ⁇ ⁇ ) ⁇ J 0 ⁇ ( ⁇ ⁇ ( ⁇ - y ) 2 + x 2 ) - ( 1 + k s ⁇ ⁇ ⁇ - k s k f ) ⁇ e - 2 ⁇ ⁇ ⁇ ⁇ h f + ( 1 + k s ⁇ ⁇ + k s k f ) ⁇ ⁇ d ⁇ ( 19 )
- ⁇ is the thermal interface conductance.
- Equation (19) denigrates to Eq. (18).
- Equation (19) denigrates to Eq. (18).
- the result shows a ⁇ 40% increase in peak temperature rise.
- this rise is not significant enough to affect any of the major conclusions associated with this study. Namely, that at the powers used to induce trenches by Tc-flow, temperature rises are small. Similarly, the qualitative aspects and scaling laws associated with the Tc-flow modeling are unaffected by these minor corrections.
- ⁇ is the surface tension, which usually linearly depends on the temperature rise (i.e.,
- FIG. 13 shows representative images at various points in the evolution of trenches ( FIG. 3 c shows cropped images associated with the second trench from the left).
- trenches were shallow ( ⁇ 1 nm deep) and characterized by slight ridges in the Tc-resist on each side of the SWNT, over time evolving into fully formed trenches, which grow and eventually (hours) began to interact with trenches from adjacent SWNTs, limiting further growth.
- Data associated with analysis of time dependence was restricted to durations where trenches were isolated from one another ( ⁇ 2 hr).
- the trench width, W Tc was defined as the width between the peak of the pile-up on either side of the trench (actual minimum widths, evaluated at the base of the Tc-resist, were much narrower). Analysis to determine the left and right side of the trench was performed in MATLAB, and involved identifying the first location to the left and right side of the trench where the slope fell below a certain threshold, 5 ⁇ 10 ⁇ 11 .
- FIG. 14 b shows cross-sectional profiles associated with the central trench at various points in the trench evolution and the identified left and right positions.
- FIG. 15 b shows the predicted W Tc based on modeled trench profiles (peaks in h ( x , t )), which also fit well to a power law with exponent of 0.25 (The parameter A depends on various Tc-resist properties, several of which are unknown).
- FIG. 15 c,d show the predicted W Tc for power densities varying from 8.3-33.3 ⁇ W/ ⁇ m for long durations and for durations that yield trench widths associated with those typical for TcEP. (Comparison to model can be difficult given the uncertainty in materials properties for the Tc-resist. Because t is normalized with respect to ⁇ and ⁇ , which are unknown, it is not possible to compare directly to t.
- W Tc is only normalized by h 0 , so it is meaningful to compare modeled t to ranges of experimental t that yield trenches of similar size to those measured experimentally).
- W Tc varies with power density. At durations associated with experimental conditions, however, almost no variation is predicted. This relative insensitivity to power is consistent with experimental observation ( FIG. 2 e,f ), where only ⁇ 20% variations in W Tc are typically observed. Such variations likely result from local changes in film viscosity associated with heating, or other effects not explicitly included in the model.
- the heated tip (radius ⁇ 100 nm) was fabricated from doped single crystal silicon, and is capable of reaching temperatures of 1000° C. with a temperature calibration to within 5° C. for this entire range. 18 Previous studies of viscous mass flow from a heated tip to a substrate revealed thermocapillarity to be an important driver of flow 19 .
- FIG. 16 shows Tc-resist layer deformation induced by tip heating for tip-substrate temperature differences between 5-45° C.
- Tc-resist material There are numerous characteristics that are critical for an effective Tc-resist material. Basic requirements are that the material can be easily deposited in thin film configurations, where vacuum deposition is preferable to spin coating, since it easily yields uniform film thickness even in regions near the partial gate electrode structures, where substrate topography is highly nonuniform. The films must afford good coverage and adhesion to both the SWNT and the quartz substrate, and at the same time be sufficiently impermeable to O 2 /CF 4 plasma to act as an effective etch resist.
- the unique chemistry of the Tc-resist material studied here combines hydroxyl and phenyl moieties which provide compatibility with both SWNT and oxide substrates.
- FIG. 18 a,b,c shows AFM associated with trench formation experiments with arrays of SWNTs and Tc-resists consisting of thin films of paraffin, TCNQ, and pentacene (similar results were achieved for TCTA and F4-TCNQ). All of these materials show features that roughly correlate to underlying SWNT heaters, but showed massive variations in resulting trenches over the area of the film.
- thermocapillary flow suggest that such behaviors are due to either low temperature coefficients of surface tension or high viscosities. While both parameters play an important role in the flow (and ⁇ 1 also plays a role in the trench profile), most materials exhibit ⁇ 1 between ⁇ 0.05 and ⁇ 0.15 mJ/m 2 /° C., while viscosities can vary by many orders of magnitude. It is likely, then, that viscosity is the most significant parameter that determines whether materials yield trenches in experimentally practical time scales and with low power levels, without significant background heating.
- Tc-resist demonstrated here meets all of these criteria, advances could be obtained through the development of materials with similar properties but also with the ability for use at smaller thicknesses (e.g. 5-10 nm, rather than 25 nm). Reductions in thickness enable decreases in W Tc (linearly with h f ) which, in turn, could allow application to arrays of SWNTs with high densities.
- thermocapillary flow is critical to the success of TcEP, because it allows uniform trenches in arrays of SWNT that incorporate significant variations in power densities among the various SWNTs.
- approaches like TcEP which rely on processes such as sublimation or ablation, their robust operation is limited by the existence of a critical temperature, T C . In such cases, at temperatures below T C , the resist will remain, while at temperatures above T C , the film will be removed.
- Thermal models can provide key insights into the scaling of this type of process.
- FIG. 19 a,b show temperature profiles and thermal gradients for a range of power densities similar to those measured experimentally. Both peak power and peak gradient scale linearly with power.
- FIG. 19 a A width associated with a process that relies on a critical temperature, Wc, can be determined ( FIG. 19 a ).
- FIG. 19 c,d show the predicted scaling for processes associated with critical temperatures of 2-10° C. It reveals that these processes yield no trench until a certain power density is reached. Afterward, the width increases dramatically with increasing power, to widths that would expose other SWNT in arrays of densities >0.1 SWNT/ ⁇ m. This type of scaling is incompatible with desired operation. For higher T C the range of powers that yield practical trench widths (several hundred nm) becomes larger. Here, the required power density to initiate trenches grows dramatically, which is also highly undesirable.
- non-ideal behavior in TcEP can occur if a trench associated with an m-SWNT exposes an s-SWNT in close proximity.
- the trench width provides an indicator for the density at which this type of behavior can be expected.
- the maximum density is defined roughly by the average trench width (trench width measured at the base of the Tc-resist ⁇ 100 nm, where W Tc is ⁇ 250 nm).
- FIG. 20 shows an AFM, cross-sectional height profile, and associated 3D renderings of such trenches.
- isolated trenches are observed along the entire lengths of the trenches ( ⁇ 8 ⁇ m).
- FIG. 21 a shows five 1 ⁇ 5 sets of SWNT processed by TcEP in this manner.
- FIG. 21 b,c show the transfer characteristics for one of the arrays before and after TcEP and the characteristics for all five devices after TcEP.
- Table 2 summarizes the results for all five arrays. For each array, after removing interconnects (lithography and etching), all of the devices (25 total) showed high on/off ratios (>1 ⁇ 10 3 ). This result demonstrates the effectiveness of performing TcEP over large areas using this type of interconnection scheme.
- FIG. 22 a shows optical micrographs associated with this process.
- a common source electrode is used to perform TcEP on two arrays with interconnected drain electrodes. Following TcEP, the gate and dielectric layers were removed.
- the load TFT the associated source and drain electrodes for TcEP served as electrodes for the final device.
- the driver TFT the source electrode was extended to yield a reduced channel length ( ⁇ 3.5 ⁇ m).
- new dielectric (SOG/HfO 2 , 35/20 nm) and top gate (Ti, 70 nm) structures were defined, to complete the fabrication.
- FIG. 22 b,c shows the electrical properties of the driver and load TFTs.
- the measured voltage transfer curve (VTC) is consistent with that predicted from load line analysis ( FIG. 23 ). Some variation between the measured and predicted VTC curves results from hysteresis in the load and driver TFTs.
- this demonstration hints that the TcEP scheme can be important for short channel devices and their circuit demonstrations.
- short channel devices such as the one shown in FIG. 6 c
- Solution of the three-dimensional Poisson equation captures the effect of contact dimensions on the electrostatics of PG-FETs; whereas solution of the drift-diffusion equation describes one-dimensional carrier transport along the s-SWNT.
- simulation also considers acceptor doping to capture the influence of (oxygen and water induced) negatively charged interface defects.
- FIG. 24 b shows measured and simulated drain to source current (I DD ) vs. gate voltage (V G ) characteristics of the PG-FET at different source-drain bias (V DS ) for the averaged s-SWNT response.
- the simulation shows good agreement with the values measured under similar bias conditions. (Measured values represent average characteristics for ⁇ 30 PG-FETs with ⁇ 200 s-SWNTs). As shown in FIG.
- the variation reflects the uncertainty of device parameters used in the simulation.
- the average mobility throughout the devices, averaged over the quasi-Fermi level variation 22 i.e.,
- ⁇ avg ⁇ ( V DS , V GS ) ⁇ 0 L ⁇ ⁇ ⁇ ( x ) ⁇ ⁇ d Q Fp d x ⁇ ⁇ ⁇ d x ⁇ 0 L ⁇ ⁇ d Q Fp d x ⁇ ⁇ ⁇ d x ( 22 ) is summarized in FIG. 24 d .
- ⁇ avg ⁇ 960-1050 cm 2 /V-sec.
- ⁇ ⁇ ( x ) ⁇ peak 1 + ⁇ peak ⁇ ⁇ d Q Fp d x ⁇ / v s ( 23 )
- ⁇ peak is the peak mobility
- V is the potential at x
- v s is the saturation velocity.
- This position dependent mobility (Eq. (23)) is later used for calculating average mobility ( ⁇ avg ) at a particular V GS , V DS using 22 .
- FIG. 25 a shows an AFM image of the channel region of the BSGS (bottom split gate structure) immediately after transfer of an array of SWNTs.
- the AFM image in FIG. 25 b shows selective formation of trenches for the entire channel length of 30 ⁇ m, by thermocapillary flow in an overlying layer of Tc-resist.
- the Tc-resist is removed with acetone.
- the resulting channel appears in FIG. 25 c .
- the red arrow highlights a pair of s-SWNTs throughout this process.
- a BSGS was used for a 1 st TcEP cycle and then cleaned by O 2 plasma treatment (200 mTorr, O 2 20 SCCM, 100 W, 20 min, Plasma-Therm RIE). New, as-grown arrays of SWNTs were then transferred to the same BSGS using PVA/thermal tape. After a 2 nd TcEP process, we observed expected operation, as shown in FIG. 26 b . The behavior of the BSGS in this second cycle was the same as for the first. The successful multiple operation was confirmed by electrical measurements before and after TcEP for each of the two arrays of SWNTs, as summarized in FIG. 5 b, c.
- I-V characteristics after TcEP reveal the maximum achievable current levels, as well as any variations in device switching behavior at high drain biases.
- FIG. 29 b indicates that the maximum output current which can be extracted is ⁇ 1.5 mA.
- FIG. 10 is measured for a device that has a single semiconducting nanotube as a channel. Simulation of I DS -V GS for this device is consistent with a diameter of 1.74 nm (directly estimated by analysis of Raman spectra) for the SWNT and matches the on/off ratio measured by varying overlap length (L OV ) and V DS ( FIG. 32 c ).
- band bending ( FIG. 33 a ) and BTBT increases at larger
- Photolithography (AZ 5214 positive photoresist), electron beam evaporation (0.6 nm Fe; AJA), and subsequent liftoff defined regions of catalyst in the geometry of strips oriented perpendicular to the preferred growth direction on ST-cut quartz substrates (Hoffman).
- the purified SWNTs i.e. consisting only of s-SWNTs
- a patterned layer of photoresist AZ 5214
- Etching in buffered hydrofluoric acid BOE 6:1, 30 s
- Stripping the photoresist completed the process.
- phase shift lithography 27 electron beam evaporation (2 nm Ti, 25 nm Pd), and lift-off (facilitated by brief ultrasonication, ⁇ 1 min) defined a narrow gap separating new source and drain electrodes on a purified array of s-SWNTs.
- PDMS stamps for phase shift lithography were cast and molded (Dow Corning, Sylgard) from a Si master, fabricated by photolithography (AZ 5214) and Bosch etching (etch/passivation, cycle time: 5 sec/5 sec, RIE power: 20 W/0 W, 35 sccm SF 6 , 110 sccm C 4 F 8 , for constant ICP power of 600 W, etch rate: 1 ⁇ m/80 s) in SF 6 to a depth of ⁇ 1 ⁇ m.
- source and drain electrodes were further defined by photolithography (AZ 5214) and a combination of wet and dry chemical etching (Transene Pd etchant, 50 sec, followed by RIE, 40 sccm CF 4 , 1.2 sccm O 2 , 150 W, 30 sec; Plasma-Therm).
- Gate dielectric layer ( ⁇ 30 nm) of HfO 2 was deposited by electron-beam evaporation followed by atomic-layer deposition of HfO 2 (10 nm).
- TcEP yielded two purified arrays of s-SWNTs.
- Previously described procedures 28 yielded SOG/HfO 2 (35 nm/20 nm) dielectrics for both transistors.
- Photolithography (AZ 5214), electron beam evaporation (100 nm Ti) and lift-off defined the gate electrodes.
- SWNTs Aligned single walled carbon nanotubes
- CVD high dynamic range RF electronics, low-noise linear amplifiers, mixed-signal devices and sensors.
- metallic and semiconducting tubes naturally grow at a statistical ratio of 1:3.
- Many electronic device applications require arrays of purely semiconducting SWNTs.
- the presence of metallic nanotubes degrades the device electronic properties, impeding high performance.
- NTF nanoscale thermocapillary flow
- Microwave initiated NTF, laser initiated NTF, and direct laser ablation were assessed in detail. We have shown while all the techniques offer promise, microwave excitation is the most promising and can provide an efficient, highly scalable path for selective heating of metallic tubes via NTF enabling highly selective removal of the metallic tubes (99.999%). To build on this, we also suggest “scale-up” of direct microwave heating of m-SWNTs to initiate thermo-capillary enabled purification (TcEP) to enable purification of aligned 3 inch SWNT wafers.
- TcEP thermo-capillary enabled purification
- FIG. 34 shows the general technical results based on the initial program objectives.
- the primary project goal was to identify a scalable technology which would enable selective elimination of metallic SWNTs.
- Three removal strategies were explored. Microwave initiated thermocapallary enabled purification (TcEP), laser initiated TcEP, and direct laser ablation, were studied in detail both experimentally and theoretically to assess the technical feasibility and scale-up potential of each. Results indicate that all of the techniques offer some level of selectivity enabling elimination of metallic tubes from the nanotube die.
- the key finding of the program was that the microwave approach, due in part to our ability to use metal-semiconductor interactions to manipulate properties of the tubes during processing, shows in essence, perfect selectivity, i.e., complete removal of metallic nanotubes with no semiconducting nanotubes damaged or destroyed. This suggests it as the best scalable choice for full wafer processing.
- CNT transistors are an important technology area and have demonstrated an ability to provide highly linear, high performance device operation at low drive voltages.
- Technology development is enabled using single walled carbon nanotubes (SWNTs), an extremely powerful and versatile class of nanomaterials with a wide array of interesting electrical properties [1]. They are being explored as the base paradigm for a wide range of device applications ranging from high performance RF electronics to flexible electronics [2].
- SWNT transistors are being pursued for applications that include low-noise linear amplifiers and mixers where low power and high spurious-free dynamic range are very important.
- Aligned submonolayer films of SWNTs, grown by CVD represent the most promising materials platform for the applications under consideration.
- thermocapillary enhanced purification (TcEP) was initially developed using selective joule heating of the m-SWNTs, detailed experimental and theoretical study has now shown that we can use other mechanisms to generate the selective heating needed to drive the underlying thermophysical phenomena.
- TcEP thermocapillary enhanced purification
- direct ablation of metallic nanotubes we have concluded that microwave initiation is an optimal purification approach enabling 100% purity.
- SWNTs single walled carbon nanotubes
- CNT carbon nanotube
- logic devices CNT-based logic devices
- CNT field-effect transistors CNT field-effect transistors
- Nanotube electrical purification is one of the fundamental barriers to technology development. This effort has removed the purification issue from the list of process needs.
- FIG. 35 A basic description of the joule heating based TcEP process is illustrated in FIG. 35 (left).
- 1) aligned arrays of SWNTs are fitted with removable electrodes to enable selective heating.
- 2) A thermal resist material is then deposited on the array and a bias current is applied to the array structure resulting in joule heating in the metallic tubes.
- Thermocapillary action induces the formation of an opening, i.e., a trench above the metallic tubes. This in essence, de-protects the metallic tubes allowing them to be selectively and exclusively removed in a simple O 2 etch process, completely compatable with conventional processing.
- FIG. 35 b shows one simplified process example.
- a simple electromagnetic field is used to selectively heat the metallic tubes.
- Our work shows that differences in the electromagnetic absorptive properties of metallic and semiconducting tubes can be exploited for initiation leading to simplified purification.
- Feasibility and demonstration of CVD array purification has been achieved using both microwave and laser initiation, moreover direct laser ablation was explored.
- the work shows that the microwave technique is superior and has the most obvious path to manufacture, yet both options hold great promise. More generally the primary program deliverables are as follows:
- FIG. 36 shows the current microwave processing path.
- the thermal resist coated, aligned tube samples are mounted in a single mode microwave reactor and exposed at a fixed time and power.
- the radiation initiates the NTF de-wetting process with trenches opening above the metallic tubes which are subsequently removed using a O 2 plasma etch.
- the current process uses dipole antenna structures deposited onto the nanotube substrates to enhance the microwave field intensity at the tube positions, ( FIG. 37 ). This feature allows for optimization of the NTF process at reasonable reactor powers.
- FIG. 37 shows a SEM image of the antenna structure with the SWNTs in the gap (right).
- An image of the microwave field distribution and a plot of the field intensity vs antenna gap width is also shown, (left, top). This gap can be tuned for process efficiency (left, bottom).
- FIG. 38 The two common geometrical variations of the microwave process that have been developed are shown in FIG. 38 . They simply differ in how the microwave antenna engages the die.
- the electrodes are deposited directly on the tube substrate.
- This “contact case” geometry has consistently produced the lowest processing power thresholds, has resulted in the highest performing transistors, and has given us the largest processing window. All the devices, (both single and multi-tube), show high on/off current ratios, (10 3 -10 4 or greater).
- the second geometry shown in FIG. 38 (scheme 2), is the “non contact” case. While this technique has proven to be more versatile, it clearly requires more microwave power.
- FIG. 39 shows the contact case (Ti antenna).
- the selectivity cure shown indexes every tube on an exposed die and simply measures the trench depth at that location. A sharp step indicates high selectivity.
- the fraction of tubes forming “deep” trenches should be ⁇ 33% consistent with expected growth statistics. Shallow trench formation is the hallmark of heating either semi-metallic or “doped” semiconducting tubes.
- FIG. 40 (bottom) illustrates the difference between antenna using Ti (41 nm) and those predominately Pd (40 nm), (Ti (1 nm)) is used here as a very thin adhesion layer). It should be noted that trenches need to be of order 15-20 nm to “de-protect” via reactive ion etch. Titanium shows some of the best results, but selectivity is sufficiently high using many metallic contact materials tested. FIG. 40 is a summary of the Ti contact results. Important to note is; a) the sharp distinction, i.e., you either get trenches or no trenches in Ti.
- FIG. 41 shows the results for 3 metals with varying work functions. (Note: samples were studied using Pd (5.1 eV), Ti ( ⁇ 4.33 eV), Mo (4.36 eV), Al (4.06), and Mg (3.7 eV) as the electrode materials. All early samples were Ti(1 nm)/Pd(40 nm) work function ⁇ 5.0 eV.) The data shows the effect is clearly not due to work function only.
- FIG. 42 (left) indicates that something more subtle is in play. Here Mg electrodes are used, lowering the work function by more than 500 meV beyond Ti.
- FIG. 42 shows trenches formed on the same sample, (the resist was deposited twice), where we have simply applied a low DC bias directly to the Ti antenna electrodes.
- Our model suggests that a work function of ⁇ 4.3 eV results in barriers for both hole and electron transport ( FIG. 42 ), resulting in all non-metallic SWNTs behaving as intrinsic materials.
- FIG. 45 shows the basic flow of the process. This approach enables processing of substrates without adding “permanent” antenna electrodes. As stated above, this approach requires more power from the microwave reactor but may offer more in device design flexibility.
- FIG. 46 shows a direct comparison between the contact and non-contact cases. Here, we have used 10 nm of SiO 2 as the spacer layer. In this experiment Ti/Pd electrodes are used for both, with identical exposure conditions. Deep trenches are formed in both cases. The non-contact structure does have a few shallow features, consistent with the need for higher exposure power. This is a limiting case, (i.e., minimizing d).
- the antenna mask is applied to the die using PDMS pillars as spacers (left image).
- the d in this case ranges from 500 nm-1 ⁇ m.
- the exposure results are shown in the micrographs.
- the middle image shows the result of a processed array die.
- the transfer function for a typical array transistor using this process is shown at the far right.
- array transistor device performance far exceeds the program goals.
- the model for the non-contact case is shown in FIG. 48 . As shown (bottom left), the model fits our observed trench profiles.
- the difference in mechanism here from the contact case is the heating current is generated from capacitive coupling of the microwave field to the metallic nanotubes.
- FIG. 49 shows optimization can be driven by contact case results.
- FIG. 50 shows an example of a structure used. Shown is a simple electrode structure with 40 micron legs and 10 micron gaps. The structure acts as a series of coupled microwave transmission lines, with the spatial field pattern amplitude and phase dependent on the size and spacing of the whole structure.
- the SEM image on the right shows the structure deposited on a nanotube die. The tubes can easily be imaged in the gaps.
- FIG. 51 shows typical trenches formed with this basic structure. The top image is a SEM of one of the typical gap positions. Below are atomic force images of gap sections.
- the scan range on the AFM is limited to about 90 microns.
- the statistics of the trenches formed is consistent with that expected, ( ⁇ 33% trench formation).
- Other interdigitated designs can be used and the metal used may be modified to control the e-field pattern and overall power. To fully optimize this approach in a processing environment one must design the lithography mask structure for the RF device envisioned, integrating the antenna design into the device layout footprint.
- FIG. 52 The basic process schematic is shown in FIG. 52 .
- FIG. 53 Shown is the density of states for both types of nanotubes along with the absorption spectra, (bottom).
- the bottom right plot shows the spectra on an expanded scale. In the wavelength range from 2-6 microns the semiconducting tube absorption is minimized relative to their metallic counterparts. We chose to operate at 2.5 microns because it was a point where the difference is large and the resist absorption is still very low.
- FIG. 54 The exposure set-up is shown in FIG. 54 .
- the nanotube die was exposed in an inverted geometry with the laser beam raster scanned along the die surface.
- the image on the right shows the exposure area with the exposure conditions described.
- the laser was focused to a diffraction limited spot and scanned to a length of 60 microns. (The absorption is ⁇ 0.01 for a layer of nanotubes at current densities, ( ⁇ 1 tube/micron)).
- FIG. 55 shows an AFM of an exposed die, as expected we form trenches above the metallic nanotubes, (verified using scanning Raman micro-spectroscopy).
- FIG. 56 shows an AFM of an exposed die, as expected we form trenches above the metallic nanotubes, (verified using scanning Raman micro-spectroscopy).
- FIG. 56 Statistical data on a number of nanotube die are shown in FIG. 56 .
- FIG. 57 shows a typical transfer function for fully processed transistors from the die. Again we obtain devices which operate well beyond the program target. Fully optimized, IR irradiation could be a viable processing option.
- selectivity is the selectivity. Thus far while small in number, we do generate trenches among a few semi-metallic and doped semiconducting tubes. These tubes of course are lost upon etching. It may be possible to treat substrates to mitigate this issue.
- FIG. 60 shows an SEM image of an ablated array. Clearly some of the individual tubes are destroyed while others remain intact. (Note: at very high power, “all” tubes ablate.)
- FIG. 61 shows a direct power mapping of the ablation process at two different IR wavelengths, (3 ⁇ m and 3.5 ⁇ m). Here fluencies range from 10's to 100's of ⁇ Watts with near diffraction limited focus. Interestingly here the higher energy, i.e., shorter wavelength excitation seems to have a better ablation range, albeit with a higher threshold power. The extent of selectivity of the ablation mechanism is modest.
- the density of tube die which can be purified with this method will be determined by the width of full trenches.
- deep trenches range from 100-500 nm. At current CVD grown wafer densities (1-2 tubes/micron), this is sufficient.
- low molecular weight polymers including but not limited to (polystyrene) PS and (polymehtylmethacrylate) PMMA, which offer promise.
- polystyrene polystyrene
- PMMA polymehtylmethacrylate
- thermocapillary (Tc) resist was chosen for its thermal properties, resistance to RIE, as well as its ability to form a uniform and continuous ultrathin film on SWNTs and quartz.
- Tc thermocapillary
- the beam was rastered back-and-forth over a distance of 60 ⁇ m orthogonally with respect to the direction of the aligned SWNTs at a rate of 0.4 ⁇ m/s.
- Selective IR absorption caused heating in the m-SWNTs, inducing thermocapillary flow of the Tc resist and forming trenches.
- RIE was used to etch m-SWNTs exposed after trench formation and the Tc resist was removed using organic solvents.
- FIG. 69 a shows the percentage of SWNTs that yielded trenches with respect to these two parameters.
- Blue represents little to no trench formation
- green shows 40-60% trench formation
- red indicates trenches formed by nearly all SWNTs. These are abrupt transitions between these regions, suggesting the presence of three distinct, stable conditions.
- heating is not sufficient for any trench formation.
- m-SWNTs are heated to sufficient temperatures to allow for trench formation.
- s-SWNTs also form trenches.
- Exposures in which the beam is rastered for 20 minutes or longer have little effect on selectivity. Trenches may become deeper or wider, but few new trenches form again suggesting stable, but phenomenologically distinct states.
- An optimal exposure condition of 3.5 mW laser power rastered for 120 minutes were chosen for all subsequent experiments.
- FIG. 69 b shows an AFM topography image of an isolated patch of aligned SWNTs on quartz patterned using photolithography and oxygen plasma etching. This patterning facilitates ease of counting.
- FIG. 69 c shows the same device after Tc resist deposition and laser exposure at the optimized condition. Trenches have been formed by some, but not all SWNTs, indicating selectivity. Trenches formed are approximately 20 nm deep and 100 nm wide. Variation in trench length arises due to Gaussian power distribution of the laser beam profile and differences in absorption coefficient among species of m-SWNTs.
- FIG. 69 d shows a higher magnification AFM scan of an exposed substrate, from which we can observe continuous and highly selective trench formation.
- SWNTs presumably m-SWNTs, form continuous trenches in the exposed area while the remaining s-SWNTs do not exhibit any such trench formation.
- the variation in trench width along each SWNT can be attributed to the presence of small defects or kinks.
- Raman spectroscopy was conducted to determine diameter and electronic type of SWNTs. Aligned arrays of SWNTs were grown on R-plane cut sapphire via CVD. Using sapphire substrates allows for direct assessment of SWNT diameter using the radial breathing mode (RBM) peaks, which are usually obscured by vibrational modes in quartz.
- RBM radial breathing mode
- FIG. 69 e AFM images of a substrate before processing (top) and after Tc resist deposition and exposure (middle) correspond perfectly with its Raman spectra (bottom). Red and blue lines correspond to RBM signals from m-SWNTs and s-SWNTs, respectively.
- Raman signals from 39 individual SWNTs were acquired, 11 from m-SWNTs, and 28 from s-SWNTs.
- Trenches formed by s-SWNTs could be caused by bundling with m-SWNTs, small band gap s-SWNTs, or heavily doped s-SWNTs.
- I-V measurements conducted before and after exposure for a representative device are shown in FIG. 69 f .
- Current retention for this particular device was 95.4%.
- FIG. 70 a An optical microscopy image of the device is shown in FIG. 70 a .
- Representative transfer and output characteristics of a representative device are shown in FIGS. 70 b and 70 c respectively.
- I ON /I OFF ratio >10 5 indicates complete removal of m-SWNTs.
- Switch ratios of 12 devices are plotted vs. the on-state currents.
- I ON /I OFF of no less than 10 4 are measured and some devices exhibit switching ratios of nearly 10 6 .
- FIG. 71 shows processes and outcomes of the microwave-based purification of large-area arrays of aligned SWNTs.
- a second patterning defines the microstrip antennas (typically Ti) over the substrate which serve as means to selectively transfer the microwave radiation energy into m-SWNTs. The resulting heating induces the local flow of a thermo-evaporated thin layer ( ⁇ 40 nm) of thermocapillary resist (Tc-resist) and opens trenches above the m-SWNTs.
- Tc-resist thermocapillary resist
- Inset represents the schematics of the geometry for extracting the trench depth, with key parameters defined.
- the SWNT, the Tc-resist and the substrate are grey, green and blue, respectively.
- (d) Transfer curve and output characteristics for 40 transistors built with large-areas ( ⁇ 40 mm in total width) of SWNTs after the purification.
- the transistors utilize the Ti microstrip antenna as the source and drain contacts, and achieve an on/off ratio ⁇ 10 3 , and large output current-25 mA.
- FIG. 72 shows the effectiveness of the microwave-based purification process.
- FIG. 73 shows coupling between the microwave field, microstrip antenna and SWNTs, and the heating mechanism.
- 3D FEM-simulation results of the electrical field distribution around the microdipole antenna.
- the incident microwave is assumed to be a TEM mode, propagating perpendicular to the antenna plane (Z axis), and with electrical field polarized along the length of the antenna (X axis).
- the simulation indicates the enhancement of the microwave field in the gap between the two antenna arms.
- the antenna is treated as a voltage source with resistance of R o +R r , and inductance of X A .
- the SWNT is assumed to be a metallic type, with a resistivity of r s and inductance of L k per unit length.
- L c Within the contact length L c , strong coupling exists between the antenna and the SWNT, through a series of capacitance C g (geometric capacitance) and C g (quantum capacitance), and a shunt conductance G.
- C g geometric capacitance
- C g quantum capacitance
- the plot below shows the trench depth profile along the length of the SWNT extracted from the AFM image (black square) and the heating profile calculated from the circuit model (red curve).
- the heating as well as the trench depth is relatively constant along the SWNT.
- (e) Schematic illustration and AFM topography image of the case where the SWNT is in contact with one side of the antenna arms. The extracted trench depth (black square) decreases starting from the left contact, all the way to the end of the SWNT, where the trench eventually diminishes. This trend can be well captured by the FEM simulation of the heating profile (red curve), by using the resistivity of the SWNT as the only fitting parameter.
- FIG. 74 shows different metals as the antennas.
- both the m-SWNTs and s-SWNTs create trenches (h>0); while for metals with low work-functions like Mo, Ti, Mg, a distinct gap in the statistics separate the m-SWNTs from the s-SWNTs.
- the Al antenna results in a continuous distribution of the trench depth, and the gap appears when the thickness of the film decreases to ⁇ 20 nm. This suggests insufficient heating for the m-SWNTs, which is probably due to the poor wettability of Al to the SWNTs.
- the simulations match with the experimental results quite well, verifying that the Schottky barrier plays the key role for selectively heating the m-SWNT by using metals with low work-functions as the antennas.
- FIG. 75 shows microwave purification based on removable antennas.
- the 2-terminal (2T) probing method provides another simple way for driving the thermocapillary flows for purification.
- FIG. 76 a shows a schematic illustration of this process.
- the SWNTs are first grown on the quartz substrate, followed by patterning strips of metal contacts on the SWNTs. Metals with relatively low work-functions with respect to the middle gap energy of the SWNTs, as well as good wettability to the SWNTs are typically good choices for the contacts.
- T c resist thermocapillary resist
- a DC voltage is applied across the metal contacts, which selectively induces Joule heating on the m-SWNTs, causing the flows of the T c resist to open trenches above the m-SWNTs.
- FIG. 76 b shows the SEM image of a pristine array of SWNTs and the corresponding AFM topography image of the induced trenches. Such well-defined, uniform trenches along the SWNTs are the key for the successful removal.
- Statistics of the trench depth associated with each SWNT in a device with total 171 SWNTs are shown in FIG. 76 c . The depths are arranged in an ascending manner (x axis is the accumulated differential fraction of the SWNTs), showing that ⁇ 37% of the SWNTs create trenches deep enough to etch away. This statistic matches ⁇ 1 ⁇ 3 of the SWNTs being metallic types.
- FIG. 77 a is the scheme to scale up the 2T probing method for the wafer scale purification.
- interdigitated electrodes are patterned over the wafer. Those electrodes are linked together, resulting in electrical paths on the entire wafer.
- thermocapillary flow is very sensitive to the temperature, it is important to control well the substrate temperature.
- a wafer of SWNTs simultaneously run through current especially when the density of the SWNTs is high >1 SWNT/ ⁇ m
- Joule heating energy will cause the temperature rise, which may eventually result in dewetting of the Tc resist. Therefore, pulse operational mode for the voltage source needs to apply in order to get better thermal management, as shown in the image of FIG. 77 b (Sunhun, Simon, Nature Nanotechnology, 2013).
- FIG. 78 a draws out the work-function of different types of metals, with respect to the valence and conduction band of the SWNT.
- S-SWNTs are usually found to be P type, therefore metals with low work-functions can form Schottky barriers to them, which will block the current flow ( FIG. 78 c ).
- the metals with relatively high work-functions can form Ohmic contact with the s-SWNTs, which enables the current flow ( FIG. 78 b ).
- thermocapillary flows When implemented on the nanoscale, material flows driven by gradients in temperature, sometimes known as thermocapillary flows, can be exploited for various purposes, including nanopatterning, device fabrication, and purification of arrays of single walled carbon nanotubes (SWNTs).
- SWNTs single walled carbon nanotubes
- Systematic experimental and theoretical studies on thermocapillary flow in thin polymer films driven by heating in individual metallic SWNTs, over a range of conditions and molecular weights reveal the underlying physics of this process.
- the findings suggest that the zero-shear viscosity is a critical parameter that dominates the dependence on substrate temperature and heating power.
- the experimentally validated analytical models in this study allow assessment of sensitivity to other parameters, such as the temperature coefficient of surface tension, the thermal interface conductance, and the characteristic length scale of the heated zone.
- Recent reports highlight the ability to use nanoscale, thermally driven processes of pattern formation for applications in ultralow power phase-change memory 1 , nanolithography 2-4 , purification of aligned arrays of SWNTs 5 and others.
- self-aligned structures in thin film coatings form as a result of local increases in temperature induced at the positions of Si or metal nanowires 2-4 or metallic SWNTs (m-SWNTs) 1,5 .
- Some of these phenomena are reported to involve physical evaporation and/or chemical change in the films.
- data indicates a process of physical mass transport, or flow, that depends on temperature, gradients in temperature and physical/chemical properties of the film and substrate support.
- Such flows can occur in organic small molecule or polymer films at peak temperatures of just a few degrees, for sources of heat that have nanoscale dimensions.
- films coated onto aligned arrays of SWNTs undergo flow only at regions of selective current injection, and Joule heating, at the m-SWNT.
- This process creates openings that allow removal of the m-SWNTs by gas phase etching, in a manner that leaves the semiconducting SWNTs unaltered.
- a full understanding of this process is necessary for further optimization and use of this physics, not only in purification of SWNT arrays, but in nanolithography, device fabrication and other areas as well.
- thermocapillary flow process we report systematic experimental and theoretical studies that highlight, directly and indirectly, the essential aspects of nanoscale thermocapillary flows in films of polystyrene (PS), driven by Joule heating 1,5 in individual single walled carbon nanotubes (SWNTs). Quantitative agreement between experiment and theory establishes use of the models reported here for predictive assessment of the thermocapillary flow process.
- PS polystyrene
- SWNTs single walled carbon nanotubes
- FIG. 79( a ) provides a scanning electron microscope (SEM) image of an individual SWNT with a pair of metal electrode contacts.
- SEM scanning electron microscope
- the trench width (W Tc ) is defined as the distance between the ridges that form in the PS on either side of the m-SWNT 5 .
- PS Sigma Aldrich, Inc
- M w between 2.5 kg/mol to 30 kg/mol
- dissolved in toluene to form a 0.8 wt % solution
- PVDF membrane filter with nominal pore size of 0.2 ⁇ m (Whatman) to remove any particulates or polymer aggregates.
- the process involves applying a DC bias for 10 minutes while monitoring the current with a parameter analyzer (Agilent 4155C).
- An atomic force microscope (AFM; Asylum MFP 3D, tapping mode) yields images of the patterns of PS induced by thermocapillary flow. Soaking the sample in toluene, drying under a stream of nitrogen, and then baking on a hotplate (110° C. for 10 minutes) allows its re-use in multiple experiments.
- the results show that W Tc increases significantly with decreasing M w , as summarized by the red symbols in FIG. 79 d .
- the physics of this process is essentially unidirectional, such that the evolution of film thickness h(x,t) can be approximated by the one dimensional lubrication equation 5 ,
- ⁇ ⁇ ⁇ ⁇ T ⁇ ⁇ T ⁇ x is the thermocapillary stress
- ⁇ the film viscosity.
- ⁇ 0 and ⁇ 1 are taken as 47.4 ⁇ 10 ⁇ 3 N/m and 0.078 ⁇ 10 ⁇ 3 N/(mK) 9 , respectively, for all PS materials examined since ⁇ 0 and ⁇ 1 depend only slightly on M w (less than 5% change for ⁇ 0 and 20% change for ⁇ 1 with M w between 2 kg/mol to 30 kg/mol) 9 .
- the model suggests that low viscosity facilitates physical mass transport induced by spatial variations in surface tension due to temperature gradients created by Joule heating in the m-SWNT.
- the zero-shear viscosity, ⁇ can be connected to M w via the Vogel equation 7 ,
- the temperature distribution for Eq. (1) can be approximated by the surface temperature 5 of the film calculated as a result of heating of the m-SWNT, which can be written
- T ⁇ ( x ) 1 k f ⁇ ⁇ ⁇ ⁇ - L / 2 L / 2 ⁇ ⁇ d u ⁇ ⁇ 0 ⁇ ⁇ Q 0 ⁇ e - ⁇ ⁇ ⁇ h 0 ⁇ ( 1 + k s ⁇ ⁇ ⁇ ) ⁇ J 0 ⁇ ( ⁇ ⁇ u 2 + x 2 ) - ( 1 + k s ⁇ ⁇ ⁇ - k s k f ) ⁇ e - 2 ⁇ ⁇ ⁇ ⁇ h 0 + ( 1 + k s ⁇ ⁇ + k s k f ) ⁇ ⁇ d ⁇ + T 0 ( 2 )
- k s and k f are the thermal conductivity of PS and quartz, respectively
- L is the length of the SWNT
- ⁇ is the interface thermal conductance between PS and quartz
- J 0 is the 0 th order Bessel function of the
- T ⁇ ( x ) 1 2 ⁇ ⁇ k s ⁇ ⁇ ⁇ ⁇ - L / 2 L / 2 ⁇ ⁇ d u ⁇ ⁇ 0 ⁇ ⁇ Q 0 ⁇ J 0 ⁇ ( ⁇ ⁇ u 2 + x 2 ) cosh ⁇ ( ⁇ ⁇ ⁇ h 0 ) + k f k s ⁇ sinh ⁇ ( ⁇ ⁇ ⁇ h 0 ) ⁇ ⁇ d ⁇ + T 0 , ( 3 )
- Equation (1) is equivalent to a pair of coupled partial differential equations
- the Fortran solver PDE_1D_MG can be used to evaluated these equations (i.e., h 1 and h 2 ), to yield the evolution of the film thickness h(x,t).
- the scaling trends arise mainly from variations in A, which yields a power law dependence of ⁇ on M w ( ⁇ M w 1.25) 7 .
- W Tc depends strongly on O 0 .
- FIG. 81( a ) summarizes a set of results similar to those of FIG.
- FIG. 80( a ) shows a collection of measurements like those of FIG. 80( b ) , which reveal scaling with Q 0 .
- the trench width is nicely replicated with analytical solution without any fitting, thereby providing further indication that r f is, to within experimental uncertainties, entirely responsible for the observed variations.
- the results presented here indicate that effects of temperature, power dissipation and molecular weight on nanoscale thermocapillary flow all arise primarily from associated variations in viscosity.
- the sensitivity to the temperature coefficient of surface tension ( ⁇ 1 ), the thermal interface conductance ( ⁇ ) are comparatively small, for values of these parameters that lie within ranges reported in the literature 7, 9 .
- the maximum temperature gradient (dT/dx) max shows little dependence on the width of the heat source for values ranging from those corresponding to a SWNT (i.e. ⁇ 1 nm) to ⁇ 100 nm diameter range; at 1 ⁇ m, the gradient is reduced by nearly an order of magnitude.
- isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure.
- any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium.
- Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.
- ionizable groups groups from which a proton can be removed (e.g., —COOH) or added (e.g., amines) or which can be quaternized (e.g., amines)]. All possible ionic forms of such molecules and salts thereof are intended to be included individually in the disclosure herein.
- salts of the compounds herein one of ordinary skill in the art can select from among a wide variety of available counterions those that are appropriate for preparation of salts of this invention for a given application. In specific applications, the selection of a given anion or cation for preparation of a salt may result in increased or decreased solubility of that salt.
- ranges specifically include the values provided as endpoint values of the range.
- ranges specifically include all the integer values of the range. For example, a range of 1 to 100 specifically includes the end point values of 1 and 100. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.
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Abstract
Description
where E is Young's modulus, L0 is the equilibrium length, ΔL is the length change under the applied stress, F is the force applied, and A is the area over which the force is applied. Young's modulus may also be expressed in terms of Lame constants via the equation:
where λ and μ are Lame constants. High Young's modulus (or “high modulus”) and low Young's modulus (or “low modulus”) are relative descriptors of the magnitude of Young's modulus in a given material, layer or device. In some embodiments, a high Young's modulus is larger than a low Young's modulus, preferably about 10 times larger for some applications, more preferably about 100 times larger for other applications, and even more preferably about 1000 times larger for yet other applications. In an embodiment, a low modulus layer has a Young's modulus less than 100 MPa, optionally less than 10 MPa, and optionally a Young's modulus selected from the range of 0.1 MPa to 50 MPa. In an embodiment, a high modulus layer has a Young's modulus greater than 100 MPa, optionally greater than 10 GPa, and optionally a Young's modulus selected from the range of 1 GPa to 100 GPa. In an embodiment, a device of the invention has one or more components, such as substrate, encapsulating layer, inorganic semiconductor structures, dielectric structures and/or metallic conductor structures, having a low Young's modulus. In an embodiment, a device of the invention has an overall low Young's modulus.
TABLE 1 |
Thermal and Mechanical parameters used in analytical and FE models. |
Coefficient of | |||||
Thermal | Thermal | Thermal | Yong's | ||
Conductivity | Diffusivity | Expansion | Modulus | Poisson | |
Materials | (Wm−1K−1) | 10−6(m2s−1) | 10−6(K−1) | 109(Pa) | Ratio |
Si | 120(ref5) | 73 | 2.6(ref6) | 165(ref7) | 0.28(ref8) |
SiO2 | 1.3(ref9) | 0.84(ref9) | 0.50(ref10) | 64(ref7) | 0.17(ref11) |
Quartz | 6.0(ref12) | — | — | — | — |
PMMA | 0.19(ref13) | 0.11(ref14) | 50(ref15) | 3.0(ref16) | 0.35(ref17) |
Tc-resist | 0.2(meas) | — | — | — | — |
When applied to the case of DC heating (f=0 Hz), and quartz substrates, the same analytical model yields an expression for the rise in temperature of the surface of the thermocapillary resist, θ=T−T∞ where T∞ defines the temperature of the background,
Here, ks and kf are the thermal conductivity of the thermocapillary resist and quartz substrate, respectively, and hf is the thickness of the resist. This solution, which is also consistent with 3D finite element analysis (ABAQUS), suggests small increases in temperature at the SWNTs (˜2-5° C.) for power densities needed to achieve trenches. The flows arise from large associated gradients in temperature (˜20° C./μm). (See Supplementary Information,
where,
With Eq. (1) for the temperature, numerical solutions to this system yield
Application of the Purification Process
TABLE 2 |
Summary of conductance of 1 × 5 |
arrays of devices before and after TcEP. |
Array # | Ion, b (A) | Ion, a (A) | Ion/Ioff ratio | Ion, a/Ion, b (%) |
#1 | 4.50E−06 | 1.02E−06 | 1.06E+03 | 22.6 |
#2 | 5.28E−06 | 1.03E−06 | 6.60E+03 | 19.5 |
#3 | 4.99E−06 | 1.13E−06 | 2.46E+04 | 22.7 |
#4 | 7.18E−06 | 1.21E−06 | 9.26E+03 | 16.9 |
#5 | 1.20E−05 | 1.16E−06 | 4.38E+03 | 9.7 |
AVG | 6.80E−06 | 1.11E−06 | 9.18E+03 | 18.3 |
The process can be applied to arrays of SWNTs that have both local (
where
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Supplementary Information
where
is thermal diffusivity, k is thermal conductivity, ρ is density, and c is specific heat capacity. The
The boundary conditions are
(1) z=−h0 (top surface)
(2) z=0
where Q1 and Q2 satisfy
P is the total power of the disk.
(3) z=h1
(4) z=h1+h2˜∞
θh
φ(r,z)=∫0 ∞
where φ(r,z) is the original function,
Solving the above equation gives
where A and B are two unknown functions to be determined according to boundary and continuity conditions. The temperature rise is then obtained by
θ=∫0 ∞(Ae −ξz +Be ξz)J 0(ξr)ξdξ (11)
Therefore, the temperature rise in Hankel space at each layer is obtained as
Tc-resist:
SiO2 layer:
Si layer:
With BCs (3)-(6) in Hankel space, we can obtain the temperature at each layer. For example, A0 and B0 are given by
θ(r,z)=∫0 +∞ A 0[exp(z√{square root over (ξ2 +q 0 2)})+exp(−z√{square root over (ξ2 +q 0 2)}−2h 0√{square root over (ξ2 +q 0 2)})]·J 0(ξr)ξdξ (13)
The surface temperature rise of the Tc-resist is then obtained by setting z=−h0 as
θ(r)=∫0 +∞2A 0exp(−h 0√{square root over (ξ2 +q 0 2)})·J 0(ξr)ξdξ (13)
As r0→0, we obtain the temperature rise due to a point heat source as
where v0 and β0 are the Poisson's ratio and coefficient of thermal expansion of the PMMA, respectively.
where kf and ks are the thermal conductivity of Tc-resist and quartz, respectively and hf is the thickness of Tc-resist.
where ζ is the thermal interface conductance. As ζ approaches to infinity, Equation (19) denigrates to Eq. (18). For ˜100 MW/m2/K for the Tc-resist/quartz interface, the result shows a ˜40% increase in peak temperature rise. However, this rise is not significant enough to affect any of the major conclusions associated with this study. Namely, that at the powers used to induce trenches by Tc-flow, temperature rises are small. Similarly, the qualitative aspects and scaling laws associated with the Tc-flow modeling are unaffected by these minor corrections.
where γ is the surface tension, which usually linearly depends on the temperature rise (i.e.,
is the thermocapillary stress with T=T∞+θ, and μ is viscosity. By introducing the following non-dimensional terms
The Fortran solver PDE_1D_MG can be used to solve for h.
is summarized in
where μpeak is the peak mobility, V is the potential at x, vs is the saturation velocity. This position dependent mobility (Eq. (23)) is later used for calculating average mobility (μavg) at a particular VGS, VDS using 22.
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with the initial condition h(x,t=0)=h0, where h0 is the initial film thickness, and the boundary conditions h(x=±∞,t)=h0 and ∂2h/∂x2 (x=±∞,t)=0 (zero pressure). Here, γ is the surface tension, which usually depends linearly on the surface temperature T of film [i.e., γ=γ0−γ1(T−273)] where γ0 is the surface tension at 273K and γ1 is the temperature coefficient of surface tension,
is the thermocapillary stress, and η is the film viscosity. For polystyrene, γ0 and γ1 are taken as 47.4×10−3 N/m and 0.078×10−3 N/(mK)9, respectively, for all PS materials examined since γ0 and γ1 depend only slightly on Mw (less than 5% change for γ0 and 20% change for γ1 with Mw between 2 kg/mol to 30 kg/mol)9. The model suggests that low viscosity facilitates physical mass transport induced by spatial variations in surface tension due to temperature gradients created by Joule heating in the m-SWNT. The zero-shear viscosity, η, can be connected to Mw via the Vogel equation7,
where A is the structure factor, B/αf is a constant7, and T∞ is the Vogel temperature, respectively. Literature7 suggests that, B/αf˜(1620±50) K, A=1.925×10−8Mw 1.25 Pa·sec, and T∞=321.4−8.3×104Mw −1 K, both with Mw in g/mol. We use B/αf=1640 K, chosen within the range defined by the literature, but with a specific value that leads to agreement between experiment and theory for the trench width (˜0.62 μm) at T0=353K, Q0=30 μW/μm and Mw=2.5 kg/mol after 10 minutes of heating. Calculated viscosities from the Vogel equation appear as blue symbols in
where ks and kf are the thermal conductivity of PS and quartz, respectively, L is the length of the SWNT, ζ is the interface thermal conductance between PS and quartz, and J0 is the 0th order Bessel function of the first kind. Here, h0=30 nm, kf 10=0.15 Wm−1K−1 and ks 11=6 Wm−1K−1. Compared with that for the case of ζ=∞, the computed peak temperatures at the surface of the PS are only ˜35% higher for ξ=108 W/(m2K) and ˜5% higher for 109 W/(m2K), which are sufficiently small that they do not affect any of the major conclusions associated with this study. Therefore, all calculations in this example correspond to the temperature with ζ=∞, i.e.,
with h1=h, initial conditions h1(x,t=0)=1 and h2(x,t=0)=0, and boundary conditions h1(x=±∞,t)=1 and h2(x=±∞,t)=0. The Fortran solver PDE_1D_MG can be used to evaluated these equations (i.e., h1 and h2), to yield the evolution of the film thickness h(x,t). The dashed line in
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Attorney | Publication | Publication | ||||
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145-03 US | 11/001,689 | Dec. 1, 2004 | 2006/0286488 | Dec. 21, 2006 | 7,704,684 | Apr. 27, 2010 |
18-04 US | 11/115,954 | Apr. 27, 2005 | 2005/0238967 | Oct. 27, 2005 | 7,195,733 | Mar. 27, 2007 |
38-04A US | 11/145,574 | Jun. 2, 2005 | 2009/0294803 | Dec. 3, 2009 | 7,622,367 | Nov. 24, 2009 |
38-04B US | 11/145,542 | Jun. 2, 2005 | 2006/0038182 | Feb. 23, 2006 | 7,557,367 | Jul. 7, 2009 |
43-06 US | 11/421,654 | Jun. 1, 2006 | 2007/0032089 | Feb. 8, 2007 | 7,799,699 | Sep. 21, 2010 |
38-04C US | 11/423,287 | Jun. 9, 2006 | 2006/0286785 | Dec. 21, 2006 | 7,521,292 | Apr. 21, 2009 |
41-06 US | 11/423,192 | Jun. 9, 2006 | 2009/0199960 | Aug. 13, 2009 | 7,943,491 | May 17, 2011 |
25-06 US | 11/465,317 | Aug. 17, 2006 | — | — | — | — |
137-05 US | 11/675,659 | Feb. 16, 2007 | 2008/0055581 | Mar. 6, 2008 | — | — |
90-06 US | 11/782,799 | Jul. 25, 2007 | 2008/0212102 | Sep. 4, 2008 | 7,705,280 | Apr. 27, 2010 |
134-06 US | 11/851,182 | Sep. 6, 2007 | 2008/0157235 | Jul. 3, 2008 | 8,217,381 | Jul. 10, 2012 |
151-06 US | 11/585,788 | Sep. 20, 2007 | 2008/0108171 | May 08, 2008 | 7,932,123 | Apr. 26, 2011 |
216-06 US | 11/981,380 | Oct. 31, 2007 | 2010/0283069 | Nov. 11, 2010 | 7,972,875 | Jul. 5, 2011 |
116-07 US | 12/372,605 | Feb. 17, 2009 | — | — | — | — |
213-07 US | 12/398,811 | Mar. 5, 2009 | 2010/0002402 | Jan. 7, 2010 | 8,552,299 | Oct. 8, 2013 |
38-040 US | 12/405,475 | Mar. 17, 2009 | 2010/0059863 | Mar. 11, 2010 | 8,198,621 | Jun. 12, 2012 |
170-07 US | 12/418,071 | Apr. 3, 2009 | 2010/0052112 | Mar. 4, 2010 | 8,470,701 | Jun. 25, 2013 |
38-04A1 US | 12/564,566 | Sep. 22, 2009 | 2010/0072577 | Mar. 25, 2010 | 7,982,296 | Jul. 19, 2011 |
71-07 US | 12/669,287 | Jan. 15, 2010 | 2011/0187798 | Aug. 4, 2011 | — | — |
60-09 US | 12/778,588 | May 12, 2010 | 2010/0317132 | Dec. 16, 2010 | — | — |
43-06A US | 12/844,492 | Jul. 27, 2010 | 2010/0289124 | Nov. 18, 2010 | 8,039,847 | Oct. 18, 2011 |
15-10 US | 12/892,001 | Sep. 28, 2010 | 2011/0230747 | Sep. 22, 2011 | 8,666,471 | Mar. 4, 2014 |
15-10A US | 14/140,299 | Dec. 24, 2013 | — | — | — | — |
19-10 US | 12/916,934 | Nov. 1, 2010 | 2012/0105528 | May 03, 2012 | 8,562,095 | Oct. 22, 2013 |
3-10 US | 12/947,120 | Nov. 16, 2010 | 2011/0170225 | Jul. 14, 2011 | — | — |
118-08 US | 12/996,924 | Dec. 8, 2010 | 2011/0147715 | Jun. 23, 2011 | — | — |
126-09 US | 12/968,637 | Dec. 15, 2010 | 2012/0157804 | Jun. 21, 2012 | — | — |
50-10 US | 13/046,191 | Mar. 11, 2011 | 2012/0165759 | Jun. 28, 2012 | — | — |
151-06A US | 13/071,027 | Mar. 24, 2011 | 2011/0171813 | Jul. 14, 2011 | — | — |
137-05A US | 13/095,502 | Apr. 27, 2011 | — | — | — | — |
216-06B US | 13/100,774 | May 04, 2011 | 2011/0266561 | Nov. 3, 2011 | — | — |
38-04A2 US | 13/113,504 | May 23, 2011 | 2011/0220890 | Sep. 15, 2011 | 8,440,546 | May 14, 2013 |
136-08 US | 13/120,486 | Aug. 4, 2011 | 2011/0277813 | Nov. 17, 2011 | — | — |
151-06B US | 13/228,041 | Sep. 8, 2011 | 2011/0316120 | Dec. 29, 2011 | — | — |
43-06B US | 13/270,954 | Oct. 11, 2011 | 2012/0083099 | Apr. 5, 2012 | 8,394,706 | Mar. 12, 2013 |
3-11 US | 13/349,336 | Jan. 12, 2012 | 2012/0261551 | Oct. 18, 2012 | — | — |
38-04E US | 13/441,618 | Apr. 6, 2012 | 2013/0100618 | Apr. 25, 2013 | — | — |
134-06B US | 13/441,598 | Apr. 6, 2012 | 2012/0327608 | Dec. 27, 2012 | — | — |
28-11 US | 13/472,165 | May 15, 2012 | 2012/0320581 | Dec. 20, 2012 | — | — |
7-11 US | 13/486,726 | Jun. 1, 2012 | 2013/0072775 | Mar. 21, 2013 | — | — |
29-11 US | 13/492,636 | Jun. 8, 2012 | 2013/0041235 | Feb. 14, 2013 | — | — |
84-11 US | 13/549,291 | Jul. 13, 2012 | 2013/0036928 | Feb. 14, 2013 | — | — |
25-06A US | 13/596,343 | Aug. 28, 2012 | 2012/0321785 | Dec. 20, 2012 | 8,367,035 | Feb. 5, 2013 |
150-11 US | 13/624,096 | Sep. 21, 2012 | 2013/0140649 | Jun. 6, 2013 | — | — |
38-04A3 US | 13/801,868 | Mar. 13, 2013 | 2013/0320503 | Dec. 5, 2013 | — | — |
38-04A4 US | 14/155,010 | Jan. 14, 2014 | — | — | — | — |
125-12 US | 13/835,284 | Mar. 15, 2013 | — | — | — | — |
30-13 US | 13/853,770 | Mar. 29, 2013 | — | — | — | — |
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